Accept numeric integer schema bounds

[leehack]

Summary

• widen JsonInteger numeric keywords and default values from int-only to num
• preserve fractional integer schema bounds/defaults during JSON schema parsing and serialization
• keep integer instance validation strict while allowing fractional multipleOf constraints

Spec evidence

• Official draft and 2025-11-25 JSON schemas define RequestId/ProgressToken as integer, but NumberSchema numeric keywords for integer schemas use JSON number values.
• Draft changelog notes schema JSON now reflects minimum/maximum/default as numbers rather than only integers.

Validation

• dart format lib/src/shared/json_schema/json_schema.dart lib/src/shared/json_schema/json_schema_validator.dart test/shared/json_schema_from_json_test.dart test/shared/json_schema_validator_test.dart
• dart test test/shared/json_schema_from_json_test.dart test/shared/json_schema_validator_test.dart
• dart analyze
• git diff --check
• dart test
• (packages/mcp_dart_cli) dart run bin/mcp_dart.dart conformance --suite all --json

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.48%. Comparing base (52f0c9c) to head (15a5d14).

Additional details and impacted files

@@                     Coverage Diff                      @@
##           spec/default-2026-discovery     #250   +/-   ##
============================================================
  Coverage                        88.48%   88.48%           
============================================================
  Files                               47       47           
  Lines                             9353     9353           
============================================================
  Hits                              8276     8276           
  Misses                            1077     1077           

Flag	Coverage Δ
unittests	88.48% <100.00%> (ø)

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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.