build(deps): drop langchain-chroma dependency

[amirfz]

Summary

Supersedes #532 (langchain-chroma 0.2.6 → 1.1.0) — instead of bumping it, this drops the dependency entirely.

langchain-chroma added nothing sherpa-ai actually used: LocalChromaStore and configure_chroma in vectorstores.py were thin wrappers around langchain_chroma.Chroma, and chromadb (the real vector DB client) is already a hard dependency — chroma_vector_store.py's ChromaVectorStore never used langchain-chroma to begin with.

Changes

• LocalChromaStore now subclasses langchain_core.vectorstores.VectorStore directly and talks to chromadb via EphemeralClient() (or an injected client, for configure_chroma's remote-server case). Public surface is unchanged — same class name, from_folder(), similarity_search(), as_retriever() — so get_vectordb() and ContextSearch (its only real caller) need no changes.
• query_chroma.py's one-off script use is replaced with chromadb's own query API + embedding_functions.OpenAIEmbeddingFunction (langchain's Embeddings interface isn't what chromadb's collection embedding_function param expects — different protocol, worth calling out since it's an easy mismatch to introduce).
• test_context_search.py's availability check was gating on langchain_chroma being importable; updated to check chromadb instead — otherwise this real end-to-end test would have started silently skipping the moment langchain-chroma was uninstalled, without anyone noticing.
• Added a focused round-trip test for LocalChromaStore (add_texts + similarity_search, with metadata) using a small deterministic fake embedding function — there was no dedicated test for this class before.
• poetry.lock regenerated with the same Poetry version as the existing lock (2.2.1) to avoid unrelated format churn.

Verification

• Mutated add_texts to drop metadata and confirmed both new tests fail; reverted and confirmed green.
• Full suite (386 passed, 2 skipped) run for real with langchain-chroma uninstalled from the environment, not just removed from pyproject.toml.

Test plan

• pytest tests/unit_tests/connectors/test_local_chroma_store.py — new round-trip tests pass
• pytest tests/unit_tests/actions/test_context_search.py — real ContextSearch → get_vectordb() → LocalChromaStore path exercised, passes
• Full suite — 386 passed, 2 skipped, langchain-chroma not installed
• Mutation test on add_texts metadata handling — caught, reverted
• CI on this PR

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com