feat: add support for PolarDB for PostgreSQL HNSW

[cxyybless]

Summary

This PR adds a pgvector-compatible PolarDB for PostgreSQL HNSW client
to VectorDBBench. It is separate from the existing MySQL-compatible
PolarDB client and does not change the existing polardb* commands.

Supported features

• HNSW index creation and search
• PQ, SQ4, SQ8, and RaBitQ internal quantization
• CLI command: vectordbbench polardbpghnsw
• Streamlit configuration and result visualization
• Optional Graph Cache readiness handling before search benchmarks
• Reuse of the existing pgvector dependency extra

PolarDB-specific internal quantization and Graph Cache options require
PolarDB vector extension 0.8.3.1 or later.

Implementation

• Reuse the PgVector client and HNSW configuration where behavior is
  compatible.
• Add PolarDB-specific index options and validation without duplicating
  the PostgreSQL client implementation.
• Build or validate Graph Cache after index creation and for search-only
  runs when requested.
• Redact PostgreSQL connection passwords from client logs.
• Add PolarDBPG configuration, lifecycle, CLI, and frontend tests.

Testing

• Python 3.11.15:
  • make lint
  • existing dataset unit test
  • 76 PolarDBPG and client-resolution tests
• Python 3.12.13:
  • make lint
  • existing dataset unit test
  • 76 PolarDBPG and client-resolution tests
  • wheel build with pip wheel --no-deps .
• PolarDB for PostgreSQL 16.14 with vector extension 0.8.3.1:
  • full 50K x 1536D load and RaBitQ 8-bit HNSW index build
  • Graph Cache reached ready and usable=yes before search
  • Recall@100: 0.9909
  • search-only runs with Graph Cache lifecycle enabled and skipped
  • plain, PQ, SQ4, and SQ8 HNSW index creation and query smoke tests
  • verified that logs and result files do not expose the database password

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: cxyybless
To complete the pull request process, please assign xuanyang-cn after the PR has been reviewed.
You can assign the PR to them by writing /assign @xuanyang-cn in a comment when ready.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[cxyybless]

/assign @XuanYang-cn