feat: Add full-text index type support

[xxsc0529]

Summary

Close #50

Solution Description

Supplements full-text index type support

[Copilot]

Pull request overview

This PR adds comprehensive support for full-text index parser types in the OceanBase vector client library. It expands the available full-text search capabilities by introducing new parser options and improves sqlglot compatibility for JSON table operations.

Key changes:

• Extends FtsParser enum to support NGRAM2, BASIC_ENGLISH, and JIEBA parsers alongside the existing IK and NGRAM options
• Updates FtsIndexParam to accept custom parser strings and handle None as default Space parser
• Adds 8 comprehensive test cases covering parser types, multi-field indexes, Chinese search, case-insensitivity, and complex queries
• Fixes JSON table client to manually construct JOIN nodes for sqlglot compatibility

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 10 comments.

File	Description
pyobvector/client/fts_index_param.py	Adds new parser types (NGRAM2, BASIC_ENGLISH, JIEBA) to FtsParser enum, updates param_str() to handle all parser types including custom strings, and improves documentation
tests/test_fts_index.py	Adds comprehensive test suite with 8 new test methods covering different parser types, multi-field indexes, Chinese text search, case-insensitive search, complex queries, and dynamic index creation
pyobvector/client/ob_vec_json_table_client.py	Refactors JSON table JOIN construction to manually create JOIN nodes instead of parsing comma-separated tables for better sqlglot version compatibility

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