fix: preserve numeric formats for 64-bit OpenAPI fields

[akasakariko]

Summary

• Preserve type: string for ProtoJSON 64-bit numbers while adding the numeric kind to format for int64, uint64, sint64, fixed64 and sfixed64 and Int64Value/UInt64Value wrappers
• Add NewStringSchemaWithFormat while retaining the existing NewStringSchema signature and behavior
• Add 39 table-driven regression cases spanning scalar, repeated and map values plus ordinary string and 32-bit controls
• Regenerate 25 affected OpenAPI golden YAML fixtures

Fixes #483

Testing

• The new tests failed before the production fix due to missing formats and passed afterward
• Full repository tests passed
• Plugin race tests passed
• Minimum supported Go 1.24.6 plugin build/tests/vet passed
• go vet ./... passed
• Formatting and git diff --check passed

Actual test commands:

• go test -count=1 ./cmd/protoc-gen-openapi/...
• go test -count=1 ./...
• go test -race -count=1 ./cmd/protoc-gen-openapi/...
• go vet ./...
• GOTOOLCHAIN=go1.24.6 go test -count=1 ./cmd/protoc-gen-openapi/...
• GOTOOLCHAIN=go1.24.6 go vet ./cmd/protoc-gen-openapi/...
• gofmt -l on the three changed Go source files and git diff --check