ci(infra): harden Dependabot version-bound preservation (#37510)

Dependabot has been stripping upper/lower bounds from internal
`langchain-*` deps in partner `pyproject.toml` files (e.g. #37288
reduced `langchain-core>=1.3.2,<2.0.0` to bare `langchain-core`). Locks
down the config so bumps preserve existing specifiers, and restores the
bounds it already mangled across the monorepo.

## Changes
- Add `versioning-strategy: increase` to every `uv` ecosystem block in
`.github/dependabot.yml` so future bumps move the lower bound in place
instead of rewriting the constraint.
- Ignore workspace-internal packages (`langchain-core`, `langchain`,
`langchain-classic`, `langchain-text-splitters`, `langchain-tests`,
`langchain-model-profiles`) on every `uv` block — these are editable
installs from local paths and their published constraints are
hand-curated for release, not Dependabot's to bump.
- Restore stripped bounds across all `libs/` packages — runtime
`dependencies` and every dep group (`test`, `dev`, `test_integration`,
`typing`, `lint`) — to `>=1.4.0,<2.0.0` for `langchain-core` and
`>=1.0.0,<2.0.0` for the other internal packages.

libs/core/pyproject.toml

@@ -51,7 +51,7 @@ typing = [
     "mypy>=1.19.1,<1.20.0",
     "types-pyyaml>=6.0.12.2,<7.0.0.0",
     "types-requests>=2.28.11.5,<3.0.0.0",
-    "langchain-text-splitters",
+    "langchain-text-splitters>=1.0.0,<2.0.0",
 ]
 dev = [
     "jupyter>=1.0.0,<2.0.0",
@@ -72,7 +72,7 @@ test = [
     "blockbuster>=1.5.18,<1.6.0",
     "numpy>=1.26.4; python_version<'3.13'",
     "numpy>=2.1.0; python_version>='3.13'",
-    "langchain-tests",
+    "langchain-tests>=1.0.0,<2.0.0",
     "pytest-benchmark",
     "pytest-codspeed",
 ]