fix(server): validate provider support when lowering runtime mode #60

t3_thread_configure(runtimeMode) currently persists a lower mode without checking the selected provider's advertised supportedRuntimeModes. For Muse, requesting auto-accept-edits is accepted while RuntimePolicy executes it as approval-required, leaving the reported mode different from actual approval behavior.

Hosted P2 finding: #57 (comment)

Validate the requested mode against the selected provider snapshot before mutation. Extend the existing MCP configure test with a provider advertising only approval-required/full-access, proving unsupported requests are rejected without thread or audit changes and supported lowering remains accepted. Preserve caller ceiling and lock-time lower-only/idle checks.

Deferred from #57 under the audit's P2/P3 cutoff. The existing fallback is more restrictive; no permission escalation occurs. This needs provider capability lookup and a focused test fixture rather than a few-line repair using existing coverage.