fix: reuse request body string across schema validation errors

[BOTecko]

Problem

When a payload fails schema validation, each flattened violation builds a SchemaValidationFailure and, for typical object bodies, fills ReferenceObject with:

referenceObject = string(requestBody)

In Go, string([]byte) always copies. The same fallback exists in:

• requests/validate_request.go
• responses/validate_response.go
• schema_validation/validate_schema.go (string(payload) and string(renderedSchema))

The /^(\d+)/ slice-index branch almost never runs for object bodies, so an 8 MiB body with 384 field errors copies the body 384 times (~3 GiB extra per request).

Fix

Cache the fallback conversion once per validation pass and reuse it for every violation. extractBasicErrors also converts renderedSchema once. Strings are immutable, so sharing the backing array does not change the public contract.

Testing

• Existing go test ./... passes.
• Added TestReferenceObjectCopiesRequestBodyPerViolation: 80 type mismatches on a ~256 KiB object. Before the fix there are 80 distinct ReferenceObject backings; after, there is 1.

Local ValidateSchemaBytes benchmark, average of 5 independent runs (schema warmed first):

Scenario	Current alloc	Fixed alloc	Current	Fixed
1 MiB, 10 invalid fields	12.22 MiB	3.12 MiB	5.3 ms	4.6 ms
1 MiB, 100 invalid fields	103.12 MiB	3.63 MiB	8.1 ms	4.0 ms
5 MiB, 100 invalid fields	511.13 MiB	15.59 MiB	26.3 ms	14.6 ms
8 MiB, 384 invalid fields	3.02 GiB	26.14 MiB	96.5 ms	25.2 ms

Worst case matches 384 × 8 MiB ≈ 3 GiB. After the cache it is one copy plus parse/error overhead.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (f309f59) to head (0d866a3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #317   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files          75       75           
  Lines        9192     9202   +10     
=======================================
+ Hits         9033     9043   +10     
  Misses        132      132           
  Partials       27       27           

Flag	Coverage Δ
unittests	98.27% <100.00%> (+<0.01%)	⬆️

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