Allow TextField indexes on PostgreSQL

[evstratbg]

Description

Allow TextField(unique=True) and TextField(db_index=True) when the configured database dialect is PostgreSQL.

This adds dialect-aware indexability validation and applies it consistently to schema generation and the migration schema editor. Other backends continue to reject indexed TextField declarations when generating or migrating schema. CharField behavior is unchanged.

Motivation and Context

PostgreSQL supports B-tree indexes and unique constraints on TEXT, but Tortoise currently rejects these declarations before the database dialect is known.

This addresses the TextField indexing portion of #384. The separate CharField request in that issue is intentionally out of scope.

How Has This Been Tested?

• Added asyncpg and psycopg schema-generation tests for TEXT UNIQUE and CREATE INDEX ... (text_column).
• Added SQLite rejection coverage to verify that the behavior remains PostgreSQL-only.
• Added migration schema editor tests for PostgreSQL support and non-PostgreSQL rejection.
• Ran the full SQLite test suite: 1913 passed, 148 skipped, 2 xfailed.
• Ran the affected test files: 76 passed.
• Ran Ruff format/check, Bandit, package build, Twine checks, and the documentation build.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing evstratbg:feature/postgres-textfield-indexes (52bc5ad) with develop (c5e5c4b)}