fix: Postgres JSON column fails with "Token NaN is invalid" when Agent stores message (psycopg InvalidTextRepresentation) (#11977)

* Refactor message model and validation methods

* [autofix.ci] apply automated fixes

* fix: address reviewer feedback on NaN sanitization PR

- Remove broken else branch in from_message that called file.path on str
  (fixes AttributeError when message.files contains plain strings)
- Restore # type: ignore comments for SQLModel/mypy compatibility
- Move import math to module level instead of inside method
- Add TestSanitizeJson with 13 tests covering nan/inf/nested/Decimal cases

* test: actually inject float('inf') in content_blocks test

TextContent.duration is int | None so Pydantic rejects float('inf')
at construction. Use a raw dict to bypass validation and genuinely
exercise the _sanitize_json path, then assert duration is None after
sanitization and the full JSON round-trip produces no NaN/Inf values.

* fix: resolve all Ruff CI lint errors

model.py:
- EM101/TRY003: assign exception messages to variables before raising

test_messages.py:
- D205: add blank line between docstring summary and description
- FBT003: avoid boolean positional arg by assigning to variable first
- PLW0177: replace != float('nan') with math.isnan()

* fix: D415 docstring period and I001 import sort

* style: fix comment spacing (2 spaces before inline comment)

* refactor: address maintainer feedback on NaN sanitization

- Use math.isfinite() for cleaner NaN/Inf checks
- Add docstring to _sanitize_json explaining PostgreSQL jsonb constraints
- Document redundant sanitization in serializer as a defensive measure
- Simplify NaN assertion in properties test for better clarity

* fix: remove unused math import in test

* style: match CI formatting after import removal

* fix: preserve string file paths in mixed lists

Corrected regression where string paths were lost if a message
contained both string paths and Image objects. Updated the
corresponding backend test to assert preservation of mixed lists.

* fix: relocate tests to unit and fix NaN properties validation test

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Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Gabriel Luiz Freitas Almeida <[email protected]>