Validate discovery result types

[leehack]

Summary

• require server/discover results to include resultType: complete
• reject missing, non-string, and non-complete discovery result discriminators
• add the same rule to the built-in CLI spec conformance suite

Spec context

The MCP 2026 draft DiscoverResult schema requires resultType, and the only valid complete discovery response shape uses resultType: "complete". server/discover is draft-only in this SDK, so this tightens that typed boundary without changing stable initialization behavior.

Part of #177.

Tests

• dart format lib/src/types/initialization.dart test/mcp_2026_07_28_test.dart packages/mcp_dart_cli/lib/src/conformance_runner.dart packages/mcp_dart_cli/test/src/conformance_command_test.dart
• dart test test/mcp_2026_07_28_test.dart
• dart pub get in packages/mcp_dart_cli
• dart test test/src/conformance_command_test.dart in packages/mcp_dart_cli
• dart analyze
• dart analyze in packages/mcp_dart_cli
• git diff --check
• dart test
• dart test in packages/mcp_dart_cli

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.49%. Comparing base (703a7dc) to head (fbff7e0).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           spec/elicit-request-task-compat     #254   +/-   ##
================================================================
  Coverage                            88.49%   88.49%           
================================================================
  Files                                   47       47           
  Lines                                 9358     9363    +5     
================================================================
+ Hits                                  8281     8286    +5     
  Misses                                1077     1077           

Flag	Coverage Δ
unittests	88.49% <100.00%> (+<0.01%)	⬆️

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[leehack]

Closing this draft micro PR because the RC spec work has been consolidated into a smaller reviewable stack. This change is preserved in #271 (type/schema validation); the consolidated tip is tree-identical to the previous micro-stack tip.