feat(tools): ToolOutput / ToolError + ToolResult wire serialization

[Kamilbenkirane]

Resolves #266.

Summary

Two pieces:

1. New types (src/celeste/tools.py): ToolOutput[Content] and ToolError[Content] Pydantic generics for executable tools to return structured results. Both exposed at top-level celeste namespace.
2. Fix Pydantic content lowering for text messages and tool results, per Fix Pydantic content lowering for text messages and tool results #266's directive.

Changes (per #266)

Use direct Pydantic API at the exact boundary — no celeste serialization helper, no methods on ToolResult:

• BaseModel content → content.model_dump_json() (string-content providers) or content.model_dump(mode="json") (Google)
• str content → passthrough
• dict/list/scalar → json.dumps(content, default=str) (fixes the existing Python repr bug for non-BaseModel non-str content)

Move text-modality message lowering into the modality protocol layer:

• chatcompletions and openresponses gain _chat_completions_text_messages / _openresponses_text_input_messages in src/celeste/modalities/text/protocols/<protocol>/client.py
• The lower-level src/celeste/protocols/<protocol>/tools.py modules no longer import Message or ToolResult

Cohere fix: dump messages with mode="json", serialize_as_any=True, exclude_none=True so nested Pydantic Message.content is preserved instead of dumped as {}.

Google left as-is — its inline if isinstance(content, BaseModel): content.model_dump(mode="json") already matches #266's directive.

Removes _debug_dump_wire TEMP diagnostic from protocols/chatcompletions/client.py (self-acknowledged temp code that fails bandit Medium severity check). Drops 4 unused supporting imports.

Test plan

• uv run pytest tests/unit_tests/ -n auto -q — 607 passed
• Pre-commit hooks (ruff, mypy, bandit) pass
• All Definition of Done items from Fix Pydantic content lowering for text messages and tool results #266 met:
  • Chat Completions serializes ToolResult(content=ToolOutput(...)) as JSON text ✓
  • OpenResponses serializes ToolResult(content=ToolOutput(...)) as JSON text ✓
  • Anthropic serializes ToolResult(content=ToolOutput(...)) as JSON text ✓
  • Google emits JSON-compatible object content for Pydantic tool results ✓
  • Cohere preserves nested Pydantic Message.content instead of dumping it as {} ✓
  • Plain string tool results keep existing behavior ✓
  • Focused tests cover the contract ✓

Coordination

Unblocks downstream celeste-tools — after this merges, celeste-tools can switch its celeste-ai dep from path install back to git source.

[claude]

Code review

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