fix(chroma_store): fix three bugs causing DB profile refresh and RAG-NL2SQL to fail

[yunshaochu]

Description

Fix three bugs in ChromaStore that caused the database profile refresh API and RAG-NL2SQL retrieval to fail silently.

Bug 1: delete_vector_name - collection existence check broken
list_collections() returns Collection objects, not strings. Comparing self._collection.name against Collection objects always returned False, causing the delete to be silently skipped. The refresh API would then skip re-embedding because vector_name_exists() still returned True.

Bug 2: delete_vector_name - stale collection reference after deletion
After delete_collection(), self._collection still pointed to the deleted collection object, causing "Collection does not exist" errors on subsequent operations.

Bug 3: create_collection / delete_vector_name - missing hnsw:space metadata
When recreating a collection after deletion, metadata={"hnsw:space": "cosine"} was not passed, resulting in the default L2 distance space. Since score = 1 - distance, L2 distances (300+) produced negative scores that were all filtered out by score_threshold=0, making RAG-NL2SQL always return 0 results.

Additionally, get_or_create_collection does not update metadata for existing collections, so create_collection now detects mismatched metadata and recreates the collection with correct settings.

How Has This Been Tested?

1. Set up MSSQL database (ShopDB) with 17 tables including Chinese-named tables
2. Called /api/v1/chat/db/refresh and /api/v2/serve/datasources/{id}/refresh — both now work correctly
3. Verified ChromaDB collections have correct {"hnsw:space": "cosine"} metadata
4. Verified RAG-NL2SQL retrieval returns results (previously returned 0)
5. Verified log output: [RAG-NL2SQL] Retrieved table info count: 10 (previously count: 0)
6. Ran make fmt, make fmt-check, make mypy — all passed

Snapshots:

Before fix (L2 distance, score always negative):

Query with L2 distance: dist=3.62, score=-2.62, pass_threshold_0=False
[RAG-NL2SQL] Retrieved table info count: 0, content: []

After fix (cosine distance, score positive):

Query with cosine distance: dist=0.55, score=0.45, pass_threshold_0=True
[RAG-NL2SQL] Retrieved table info count: 10, content: [...]

Checklist:

• My code follows the style guidelines of this project
• I have already rebased the commits and make the commit message conform to the project standard.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (N/A - bug fix, no doc changes needed)
• Any dependent changes have been merged and published in downstream modules (N/A)

[chenliang15405]

LGTM

[chenliang15405]

@yunshaochu Please format code with make fmt