Expand HTTP coverage and harden response API for production use

[Copilot]

The library was missing common HTTP status helpers, had unexported success functions, lacked fluent header mutation, and panicked on nil writer. This fills all those gaps.

New HTTP status helpers

Client errors (errors/client.go):

• Gone (410), UnprocessableEntity (422)

Server errors (errors/server.go):

• NotImplemented (501), BadGateway (502), GatewayTimeout (504)

New message constants added in utils/lib.go for all of the above.

Fluent header setters on Response

SetHeader and SetHeaders added alongside the existing SetMessage/SetStatus/SetData, enabling:

gecho.Success(w).
    SetHeader("X-Request-ID", requestID).
    SetHeaders(map[string]string{"X-Version": "v1", "X-Env": "prod"}).
    SetData(payload).
    Send()

Both methods are nil-safe and fully chainable.

nil writer no longer panics

writeJSON now returns fmt.Errorf("http.ResponseWriter is nil") instead of calling panic. Middleware chains handle errors gracefully rather than crashing.

Previously unreachable exports

Accepted (202) and NoContent (204) were implemented in success/common.go but never wired into the top-level gecho package. All new error helpers are also exported from the root package.

Auto-send limitation documented

buildResponse's Send() option path cannot surface write errors without a breaking API change — this is now explicitly documented in a comment directing users to the fluent Response.Send() method when error handling matters.

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