Use the precompiled server in stdio integration tests - #458

Summary

• Route the stdio integration test through the existing smokeProgram helper for the already-registered example/server_stdio.dart.
• Keep the original absolute source path when precompilation is unset or disabled; use the compiled artifact when MCP_PRECOMPILED_SMOKE=1.
• Preserve the source-existence assertion, all behavioral assertions, timeouts, handlers and cleanup. No production code, protocol, workflow, registry, dependency or SDK-floor changes.

Context

The post-merge macOS run for #457 crossed the five-second discovery budget before falling back to legacy initialization and receiving a metadata error. The identical PR tree passed on macOS; one separately approved macOS-only retry also passed. Source compilation is part of this startup budget because this test bypassed the existing compiled server artifact. The precise historical cold-compilation versus scheduling cause remains unproven.

This change removes avoidable source compilation from the precompiled CI path without extending deadlines or changing discovery/fallback behavior.

Validation

• Full SDK suite, source mode: 2,030 passed
• Full SDK suite, precompiled mode: 2,030 passed
• Focused integration/helper tests: 4/4 passed in each mode
• Unset-environment source integration: passed
• Missing-artifact negative control: failed closed before child launch; artifact restored
• Format: 322 files, zero changes; analysis: no issues; diff check: passed
• All 18 registered smoke programs compiled
• Independent exact-diff review: no findings

No release or changelog change is needed for this test-only change. Hosted CI is required before merge; merging and any workflow reruns require separate approval.