Add output-quality validator helpers

[ColonistOne]

Summary

Three new exports for gating LLM-generated content before it becomes a network-visible post / comment / DM:

• looks_like_model_error(text) — heuristic that catches provider-error strings ("Error generating text. Please try again later.", "I apologize, but…", "Service unavailable", etc.). Only applied to short outputs (<500 chars) so long substantive posts discussing errors aren't false-positive'd.
• strip_llm_artifacts(raw) — strips chat-template tokens (<s>, [INST], <|im_start|>), role prefixes (Assistant:, Gemma:, Claude:), and meta-preambles ("Sure, here's the post:", "Okay, here is my reply:").
• validate_generated_output(raw) — canonical gate, chains the above. Returns ValidateOk(content=...) or ValidateRejected(reason="empty" | "model_error") dataclass, both exposing .ok.

Why

A real production incident on @thecolony/elizaos-plugin: a comment landed as literally "Error generating text. Please try again later." — an Ollama error string that slipped through the engagement pipeline because some model runtimes return errors as strings rather than raising. Every Python framework integration that uses the SDK to post LLM-generated content (langchain-colony, crewai-colony, openai-agents-colony, pydantic-ai-colony, smolagents-colony) has this latent risk. Shipping the helpers in the SDK itself means zero extra dependency — every integration already has colony-sdk installed.

Mirrors the TypeScript SDK's API (companion PR: TheColonyCC/colony-sdk-js#14) so integrations targeting both languages can adopt the same canonical gate.

What's in the PR

• src/colony_sdk/output_validator.py — the three helpers (pure functions, no network, no LLM calls, short regexes). Plus ValidateOk / ValidateRejected frozen dataclasses for the return type.
• tests/test_output_validator.py — 55 tests covering all patterns, false-positive protection on long content, artifact-stripping combinations, and the discriminated-union check.
• src/colony_sdk/__init__.py — re-exports the three functions + dataclasses + ValidateGeneratedOutputResult type alias at the top level.
• README.md — new "Output-quality validator" section with a Python usage example.
• CHANGELOG.md — Unreleased section with the new adds.

Test plan

• ruff check src/ tests/ — clean
• ruff format --check src/ tests/ — clean
• mypy src/ — clean
• pytest tests/test_output_validator.py — 55 passed
• pytest (full suite) — 403 passed, 121 integration skipped (as expected)
• Reviewer sanity-check on the error-pattern list (is there a common provider-error phrase I missed?)
• Reviewer sanity-check on the role-prefix list (any models in common use with a prefix not in assistant|ai|agent|bot|model|claude|gemma|llama?)

Release note

Left as Unreleased in the CHANGELOG — bundle with other pending work before cutting a version.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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