fix(analyzer): a parameter serialized with content went out style-encoded

[giraffesyo]

Closes #77.

parameters:
  - name: filter
    in: query
    content:
      application/json:
        schema: { $ref: "#/components/schemas/Filter" }

convertParam read only param.Schema and never looked at param.Content, so the parameter lost its type and was encoded with the form style:

type ListItemsParams struct {
	Filter *any `json:"filter,omitempty"`   // before
}

addQueryParam(queryValues, "filter", "form", true, params.Filter)

Not merely awkward: a struct handed to the style encoder does not come out as the JSON document the server is parsing, so the request was wrong on the wire.

Now

type ListItemsParams struct {
	Filter *Filter `json:"filter,omitempty"`
}

addContentQueryParam(queryValues, "filter", params.Filter)

The value is serialized as the media type it declares, then percent-encoded into the query. Header and cookie parameters get the same treatment through setContentHeader and addContentCookieHeader.

With #80 merged, the schema behind a content parameter is typed even when written inline, so content: {application/json: {schema: {type: object, ...}}} yields a real struct rather than any. There is a test for that composition.

Decisions

• Only media types the generator can write are serialized. An application/xml content parameter keeps today's style encoding and warns, rather than sending JSON where the server expects something else:

  warning: parameter "legacy": application/xml is not a media type this generator serializes, so the value is sent style-encoded
  

  The warning rides the pkg.Warnings channel from fix(analyzer): a security scheme the generator cannot use costs the whole client #64, now also fed from an analyzer-level buffer.

• No name hint on the schema branch. Passing one there types inline unions and objects in style-encoded parameters, which sounds like an improvement and is not: the query encoder writes a synthesized union out as its Go shape (value=...) rather than as the value the server parses. TestNullableUnion_QueryParamsResolveToTheVariantType caught it. Typed union parameters need the encoder to understand them first, which is separate work.

Tests

• internal/analyzer/operations_test.go: the media type and its schema reach the IR, a style parameter carries no content type, and an unserializable media type produces exactly one warning naming the parameter.
• internal/generator/e2e_content_param_test.go: compiles and runs against httptest, decoding the query parameter and the header back into the declared type, checking an inline schema is typed and serialized, that a style parameter beside it still goes out unquoted, and that an omitted optional parameter sends nothing rather than null.

gofmt, go vet ./..., and go test ./... pass.