Add MCP 2026 RC discovery foundation

[leehack]

Summary

• add MCP 2026-07-28 RC version constants, stateless metadata helpers, and server/discover wire types
• add server-side server/discover handling and stateless request metadata validation
• add opt-in client discovery and stateless metadata propagation while keeping stable initialize behavior available
• add regression coverage for discovery, stateless request metadata, and unsupported protocol errors

Stacked PR Plan

This is the parent PR for issue #177. Follow-up PRs should target this branch until it lands.

Planned stack:

1. Discovery foundation (this PR)
2. Streamable HTTP 2026 stateless/header behavior
3. subscriptions/listen
4. MRTR / InputRequiredResult
5. task extension compatibility and dev release prep

Release Note

Do not merge or publish as stable until the official MCP 2026-07-28 spec is released. This PR is intended for RC/dev review only.

Validation

• dart analyze
• dart test test/mcp_2026_07_28_test.dart
• dart test test/server/streamable_mcp_server_test.dart
• dart test -j 1

Fixes part of #177.

[codecov]

Codecov Report

❌ Patch coverage is 98.75776% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.20%. Comparing base (d0f706c) to head (4a81430).

Files with missing lines	Patch %	Lines
lib/src/server/server.dart	97.77%	1 Missing ⚠️
lib/src/types/json_rpc.dart	93.33%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           dev/2026-07-28-rc     #178      +/-   ##
=====================================================
+ Coverage              84.92%   85.20%   +0.28%     
=====================================================
  Files                     45       45              
  Lines                   7421     7579     +158     
=====================================================
+ Hits                    6302     6458     +156     
- Misses                  1119     1121       +2     

Flag	Coverage Δ
unittests	85.20% <98.75%> (+0.28%)	⬆️

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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 #267 (MCP 2026 feature foundation); the consolidated tip is tree-identical to the previous micro-stack tip.