Add AllowReserved support for OpenAPI parameter serialization

Add the AllowReserved field to StyleParamOptions,
BindStyledParameterOptions, and BindQueryParameterOptions. When set to
true for query parameters, RFC 3986 reserved characters are left
unencoded in the serialized output, per the OpenAPI 3.x allowReserved
specification.

Changes:
- Add AllowReserved bool to all parameter option structs
- Implement escapeQueryAllowReserved for custom escaping that preserves
  reserved characters while still encoding spaces and control characters
- Thread allowReserved through internal helpers (stylePrimitive,
  styleSlice, styleStruct, styleMap, processFieldDict)
- Fix styleStruct recursive call to use StyleParamWithOptions instead
  of StyleParamWithLocation, which was silently dropping options

Partially resolves oapi-codegen/oapi-codegen#1342
Prerequisite for oapi-codegen/oapi-codegen#2183

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Add escapeParameterName helper that percent-encodes parameter names
in the prefix/separator construction of styleSlice, stylePrimitive,
and processFieldDict. This ensures characters like [] in parameter
names (e.g. user_ids[]) are properly encoded as %5B%5D in query
strings, per RFC 3986.

Note: MarshalDeepObject handles its own serialization independently
and does not yet encode parameter names in bracket notation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>