refactor(models): remove PR labels from runtime errors

[jack-scitix-ai]

Type

• refactor — code restructuring, no behavior change

Summary

• Replace internal development labels such as PR 1, PR 3, PR-5, and
  #25 in user-visible errors with stable descriptions of the actual dialect,
  binder, capability, or legacy-bypass limitation.
• Keep maintainer-only comments and TODO(PR-5) markers unchanged.
• Update capability, validation, registry, and legacy SGLang tests to lock the
  stable wording and prevent internal planning labels from returning to runtime
  diagnostics.
• This changes diagnostic wording only; request handling, response handling,
  capability admission, and runtime behavior are unchanged.

Test Plan

Automated

• Lint/format clean (ruff check && ruff format --check)
• Type check clean (ty check)
• Affected unit tests pass (896 tests)

Checklist

Required (all PRs)

• PR title follows conventional format (type(scope): description)
• No internal paths, credentials, or personal info in committed files
• AI-generated code has AI-Generated Code - <model> (<provider>) in module docstring
• No new upper-layer dependencies added to core/
• Deleted code verified — no remaining call sites depend on it