Tighten agent harness basics

[haasonsaas]

Summary

• Rewrite the README around the actual lightweight provider-adapter contract, optional SDK extras, and Bazel RBE workflow.
• Make JSON Schema generation cover Literal, None, Python 3.10 union syntax, and multi-type unions without breaking older local interpreters.
• Add validation for retry backoff, top_p, and max_output_tokens.
• Add registry isolation and coverage for schema/config basics.
• Correct the Bazel RBE workflow action pins and install uv on the self-hosted runner before Bazel checks.

Test plan

• python3 -m pytest -q
• ruff check .
• black --check .
• PATH=/Users/jonathanhaas/.codex-tools/bin:$PATH actionlint .github/workflows/ci.yml .github/workflows/bazel-rbe.yml
• PATH=/Users/jonathanhaas/.codex-tools/bin:$PATH make bazel-check

[cursor]

PR Summary

Medium Risk
Medium risk due to changes in JSON schema generation (now emitting anyOf/enum and handling PEP 604 unions) and stricter HarnessConfig validation, which could alter tool schemas and reject previously-accepted configs.

Overview
Tightens the harness “basics” by expanding python_type_to_json_schema to support None, Literal enums, PEP 604 unions (T | None), and multi-type unions via anyOf instead of collapsing to a single type.

Adds additional HarnessConfig validation for retry_backoff, max_output_tokens, and top_p, plus test isolation by auto-clearing the global tool registry between tests and new test coverage for the schema/validation edge cases.

Updates the Bazel RBE workflow by fixing action pins, installing uv, and adding an explicit make bazel-rbe-smoke step; the README is rewritten to match the lightweight provider-adapter/extras model and updated dev/RBE instructions.

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