core: fix Image prompt template hardcoded template format (#27495)

Fixes #27411 

**Description:** Adds `template_format` to the `ImagePromptTemplate`
class and updates passing in the `template_format` parameter from
ChatPromptTemplate instead of the hardcoded "f-string".
Also updated docs and typing related to `template_format` to be more
up-to-date and specific.

**Dependencies:** None

**Add tests and docs**: Added unit tests to validate fix. Needed to
update `test_chat` snapshot due to adding new attribute
`template_format` in `ImagePromptTemplate`.

---------

Co-authored-by: Vadym Barda <vadym@langchain.dev>

libs/core/tests/unit_tests/prompts/__snapshots__/test_chat.ambr

@@ -3119,6 +3119,7 @@
                   'template': dict({
                     'url': 'data:image/jpeg;base64,{my_image}',
                   }),
+                  'template_format': 'f-string',
                 }),
                 'lc': 1,
                 'name': 'ImagePromptTemplate',
@@ -3138,6 +3139,7 @@
                   'template': dict({
                     'url': 'data:image/jpeg;base64,{my_image}',
                   }),
+                  'template_format': 'f-string',
                 }),
                 'lc': 1,
                 'name': 'ImagePromptTemplate',
@@ -3157,6 +3159,7 @@
                   'template': dict({
                     'url': '{my_other_image}',
                   }),
+                  'template_format': 'f-string',
                 }),
                 'lc': 1,
                 'name': 'ImagePromptTemplate',
@@ -3177,6 +3180,7 @@
                     'detail': 'medium',
                     'url': '{my_other_image}',
                   }),
+                  'template_format': 'f-string',
                 }),
                 'lc': 1,
                 'name': 'ImagePromptTemplate',
@@ -3195,6 +3199,7 @@
                   'template': dict({
                     'url': 'https://www.langchain.com/image.png',
                   }),
+                  'template_format': 'f-string',
                 }),
                 'lc': 1,
                 'name': 'ImagePromptTemplate',
@@ -3213,6 +3218,7 @@
                   'template': dict({
                     'url': 'data:image/jpeg;base64,foobar',
                   }),
+                  'template_format': 'f-string',
                 }),
                 'lc': 1,
                 'name': 'ImagePromptTemplate',
@@ -3231,6 +3237,7 @@
                   'template': dict({
                     'url': 'data:image/jpeg;base64,foobar',
                   }),
+                  'template_format': 'f-string',
                 }),
                 'lc': 1,
                 'name': 'ImagePromptTemplate',

libs/core/tests/unit_tests/prompts/test_chat.py

@@ -31,6 +31,7 @@ from langchain_core.prompts.chat import (
     SystemMessagePromptTemplate,
     _convert_to_message,
 )
+from langchain_core.prompts.string import PromptTemplateFormat
 from tests.unit_tests.pydantic_utils import _normalize_schema
 
 
@@ -298,6 +299,77 @@ def test_chat_prompt_template_from_messages_mustache() -> None:
     ]
 
 
+@pytest.mark.requires("jinja2")
+def test_chat_prompt_template_from_messages_jinja2() -> None:
+    template = ChatPromptTemplate.from_messages(
+        [
+            ("system", "You are a helpful AI bot. Your name is {{ name }}."),
+            ("human", "Hello, how are you doing?"),
+            ("ai", "I'm doing well, thanks!"),
+            ("human", "{{ user_input }}"),
+        ],
+        "jinja2",
+    )
+
+    messages = template.format_messages(name="Bob", user_input="What is your name?")
+
+    assert messages == [
+        SystemMessage(
+            content="You are a helpful AI bot. Your name is Bob.", additional_kwargs={}
+        ),
+        HumanMessage(
+            content="Hello, how are you doing?", additional_kwargs={}, example=False
+        ),
+        AIMessage(
+            content="I'm doing well, thanks!", additional_kwargs={}, example=False
+        ),
+        HumanMessage(content="What is your name?", additional_kwargs={}, example=False),
+    ]
+
+
+@pytest.mark.requires("jinja2")
+@pytest.mark.requires("mustache")
+@pytest.mark.parametrize(
+    "template_format,image_type_placeholder,image_data_placeholder",
+    [
+        ("f-string", "{image_type}", "{image_data}"),
+        ("mustache", "{{image_type}}", "{{image_data}}"),
+        ("jinja2", "{{ image_type }}", "{{ image_data }}"),
+    ],
+)
+def test_chat_prompt_template_image_prompt_from_message(
+    template_format: PromptTemplateFormat,
+    image_type_placeholder: str,
+    image_data_placeholder: str,
+) -> None:
+    prompt = {
+        "type": "image_url",
+        "image_url": {
+            "url": f"data:{image_type_placeholder};base64, {image_data_placeholder}",
+            "detail": "low",
+        },
+    }
+
+    template = ChatPromptTemplate.from_messages(
+        [("human", [prompt])], template_format=template_format
+    )
+    assert template.format_messages(
+        image_type="image/png", image_data="base64data"
+    ) == [
+        HumanMessage(
+            content=[
+                {
+                    "type": "image_url",
+                    "image_url": {
+                        "url": "data:image/png;base64, base64data",
+                        "detail": "low",
+                    },
+                }
+            ]
+        )
+    ]
+
+
 def test_chat_prompt_template_with_messages(
     messages: list[BaseMessagePromptTemplate],
 ) -> None:

libs/core/tests/unit_tests/prompts/test_prompt.py

@@ -8,6 +8,7 @@ import pytest
 from syrupy import SnapshotAssertion
 
 from langchain_core.prompts.prompt import PromptTemplate
+from langchain_core.prompts.string import PromptTemplateFormat
 from langchain_core.tracers.run_collector import RunCollectorCallbackHandler
 from tests.unit_tests.pydantic_utils import _normalize_schema
 
@@ -610,7 +611,9 @@ async def test_prompt_ainvoke_with_metadata() -> None:
 )
 @pytest.mark.parametrize("template_format", ["f-string", "mustache"])
 def test_prompt_falsy_vars(
-    template_format: str, value: Any, expected: Union[str, dict[str, str]]
+    template_format: PromptTemplateFormat,
+    value: Any,
+    expected: Union[str, dict[str, str]],
 ) -> None:
     # each line is value, f-string, mustache
     if template_format == "f-string":