feat: support override attributes and type placeholders in .swaggo

[isasmendiagus]

Summary

• Add key:value attribute syntax to .swaggo replace directives (nullable, optional, format)
• Add $T type parameter placeholder for generic catch-all overrides
• Introduce Override struct to replace raw string type mappings

Closes #2147
Closes #1852

Changes

• parser.go — Override struct, matchOverride(), getOverrideForType(), applyOverrideAttrs(), updated getTypeSchema() and parseStructField()
• gen/gen.go — parseOverrides() with key:value attribute parsing
• parser_test.go — new tests for nullable, format, placeholder, and matchOverride
• gen/gen_test.go — new test cases for attrs, format, malformed input, and placeholder
• generics_test.go — updated to use Override struct

Usage

// Primitives with attributes
replace common.Optional[string] string optional:true nullable:true
replace common.Optional[time.Time] string optional:true nullable:true format:date-time

// Generic catch-all
replace common.Optional[$T] $T optional:true nullable:true

[maxclaus]

@sdghchj could you please check this one out when you have some time? It would be a great addition to the tool, being able to handle generic wrapper types globally is something a lot of codebases need.

I tested locally on my project and confirmed it works great. Here's a concrete example of what it unlocks.

Context: we use a tri-state optional.Value[T] wrapper for PATCH endpoints (absent / null / present), since Go's JSON marshaller does not distinguish null from absent (undefined) fields.

Before, we had to maintain a parallel *Doc struct purely for swagger:

// PatchRequestPayloadDoc is a doc-only type used to generate correct swagger output.
// It mirrors PatchRequestPayload but replaces optional.Value[T] with *T.
type PatchRequestPayloadDoc struct {
    Date             *int64            `json:"date"`
    Status           *domain.Status    `json:"status"`
} // @name PatchRequestPayload

type PatchRequestPayload struct {
    Date             optional.Value[int64]            `json:"date,omitempty"`
    Status           optional.Value[domain.Status]    `json:"status,omitempty"`
}

...
// @Param body body PatchRequestPayloadDoc true "Patch payload"

After, with a single line in .swaggo:

replace optional.Value[$T] $T nullable:true

The real struct is used directly in the annotation, no duplicate needed:

type PatchRequestPayload struct {
    Date             optional.Value[int64]            `json:"date,omitempty"`
    Status           optional.Value[domain.Status]    `json:"status,omitempty"`
}

...
// @Param body body PatchRequestPayload true "Patch payload"

Generated output renders each field as its inner type with x-nullable: true, exactly correct. Zero optional.Value-* intermediate definitions in the spec.