fix: move _transform_output to base _predict pipeline

[Kamilbenkirane]

Summary

Closes #142

• Moves _transform_output() call from each provider's _parse_content() to the base _predict() pipeline in ModalityClient, preventing silent structured output failures when new providers forget the call
• Removes redundant _transform_output() from all 11 text provider _parse_content() methods
• Updates 4 provider mixin docstrings that showed the old pattern in usage examples
• Makes _transform_output signature generic (Content instead of TextContent) to work at the base pipeline level
• Changes ParameterMapper.parse_output base signature to Any as a pragmatic bridge — tracked for proper generic fix in Make ParameterMapper generic on Content type #186

Test plan

• Added TestPredictTransformOutput with 2 integration tests verifying _predict() applies _transform_output correctly
• All 478 unit tests pass
• mypy passes clean (no type: ignore needed at call sites)
• All pre-commit hooks pass (ruff, mypy, bandit, tests)

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.