fix(responses): restore parity with chat completions and refactor parsing

[PsiACE]

Description

This PR restores behavioral parity between chat completions and responses when use_responses=true, and
improves maintainability.

What changed

• Fixed main-path parity across stream and non-stream flows.
• Completed responses streaming parsing for:
  • text deltas
  • tool-call deltas
  • usage events
• Fixed non-stream responses tool schema conversion (prevents tool-call request failures).
• Refactored parsing out of ChatClient into dedicated adapters:
  • clients/parsing/completion.py
  • clients/parsing/responses.py
  • clients/parsing/common.py
• Aligned reasoning_effort handling:
  • forwarded correctly for completions
  • mapped to reasoning.effort for responses (explicit reasoning still wins)
• Added default stream_options.include_usage=true for completion streaming when not explicitly set.

Commits included

1. d8c3349 fix: restore responses parity across main paths
2. de41fa9 refactor: split chat parsing into completion and responses adapters
3. de27115 fix: align reasoning and stream usage defaults across chat and responses