Preserve legacy Python interop helper compatibility - #446

Summary

• Keep the legacy Python fixture on its current SDK 1.x-compatible time-server helper.
• Ignore mcp-server-time versions >=2026.10.19 only in /test/interop/python: that release requires mcp>=2.3,<3 and cannot resolve with the intentional mcp<2 constraint.
• Document removal when the legacy lane retires or a compatible helper returns; leave the independent Python SDK 2.x fixture fully monitored.

Supersedes #445. No package versions, runtime dependencies, APIs, or minimum SDKs change.

Validation

• YAML parsing and structural/scoping checks passed; fixture requirements are unchanged.
• Python package version-range boundary checks passed.
• dart format --output=none --set-exit-if-changed .: 322 files, no changes.
• dart analyze: no issues.
• dart test --reporter expanded: 2030 tests passed.
• Independent review found no blocker.

The ignore deliberately excludes future versions until a maintainer confirms SDK 1.x compatibility or retires the legacy lane.