test: update MCP conformance to alpha.8

[leehack]

Summary

• Update MCP official conformance runners/docs from @modelcontextprotocol/conformance@0.2.0-alpha.7 to 0.2.0-alpha.8.
• Add alpha.8 stateless server diagnostics for missing client capabilities, response-stream shape, and request-scoped logging.
• Keep the 2026 client suite aligned with alpha.8 custom-header expectations by not mirroring the unannotated float_val parameter.
• Refresh conformance README, spec coverage, expected-failure headers, and changelog notes.

Existing issue / PR check

• No dedicated issue found in the current conformance-alpha8 search.
• Complements the TypeScript 2026 interop repin work in test: update 2026 interop and conformance pins #298; this PR is focused on the official conformance alpha.8 bump.

Test Plan

• dart format --output=none --set-exit-if-changed lib/src/server/mcp_server.dart test/conformance/mcp_2026_07_28_rc_client.dart test/conformance/mcp_2026_07_28_rc_server.dart test/conformance/run_2025_server_conformance.dart test/conformance/run_2026_07_28_rc_client_conformance.dart test/conformance/run_2026_07_28_rc_server_conformance.dart
• git diff --check
• dart analyze
• dart test --reporter compact test/docs/markdown_docs_test.dart test/mcp_2026_07_28_test.dart
• dart run test/conformance/run_2026_07_28_rc_server_conformance.dart --timeout-seconds 90 --output-dir /tmp/mcp-alpha8-server-final — 40 passed, 0 failed/timeout
• dart run test/conformance/run_2026_07_28_rc_client_conformance.dart --timeout-seconds 90 --output-dir /tmp/mcp-alpha8-client-final — 31 passed, 1 expected failure (json-schema-ref-no-deref)
• dart run test/conformance/run_2025_server_conformance.dart --timeout-seconds 120 --output-dir /tmp/mcp-alpha8-2025-server-final — 49 passed, 0 failed
• dart run tool/testing/run_ts_2026_07_28_rc_interop.dart — TypeScript preview client -> Dart server and Dart preview client -> TypeScript server succeeded

Notes

• The json-schema-ref-no-deref client scenario remains expected-failed because the upstream alpha.8 fixture still behaves like a stable-only server for that scenario.
• Local npm emitted unrelated user-level always-auth deprecation warnings while installing TS fixture dependencies; no repo/global config was changed.

[leehack]

Closing as superseded: the alpha.8 conformance changes from this PR have been folded into #298.