feat: validate maxProperties/minProperties as map entry-count bounds (#370)

[eseidel]

Summary

maxProperties/minProperties were parsed then discarded
(_ignored in parser.dart), dropping a real constraint on
map-shaped schemas (type: object + additionalProperties, no
fixed properties) — a -v regen tallies 49 sites in discord and 3 in
github. For a Map<String, T> the entry count is dynamic, so these are
the map analog of an array's maxItems/minItems, which we already
validate.

This threads both bounds through every layer
(SchemaMap → ResolvedMap → RenderMap), mirroring the existing
maxItems/minItems path, and emits a combined
validate(minProperties: ..., maxProperties: ...) call on the map
field. A new ValidateMap extension in the shipped api_exception.dart
enforces it via Map.length, matching ValidateArray.

On a fixed-property object the property count is static, so the
constraint carries no runtime meaning — it stays dropped-and-detail-
logged there (the resolver owns the semantics).

Generated output (optional map → ?., required map → .):

Lobby({required this.requiredMeta, this.metadata}) {
  metadata?.validate(minProperties: 1, maxProperties: 25);
  requiredMeta.validate(maxProperties: 5);
}

Closes #370.

Validation

• Unit tests: parser (a map carries the bounds and is not detail-logged;
  a fixed-property object drops + detail-logs them), render
  (RenderMap.validationCall combines both bounds, omits the absent
  one, is null when unconstrained).
• End-to-end repro: analyze-clean, correct ?./. receiver per
  optional/required.
• Regenerated the rotation: discord (29 files) and github (8 files) now
  emit validate(maxProperties: N) on map fields; github, discord, and
  backstage third_party packages stay analyze-clean; the
  minProperties: 1 model's generated round-trip test passes (the
  single-entry map example satisfies the bound).

[codecov]

Codecov Report

❌ Patch coverage is 60.86957% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.53%. Comparing base (1c1567a) to head (4161006).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/templates/api_exception.dart	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
- Coverage   96.67%   96.53%   -0.14%     
==========================================
  Files          25       25              
  Lines        5957     5981      +24     
==========================================
+ Hits         5759     5774      +15     
- Misses        198      207       +9     

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