feat: Context.FormValue() and Context.FormFile() for form handling

[FumingPower3925]

Summary

Add form-data parsing methods to Context:

• Context.FormValue(key string) string — returns a form field value from application/x-www-form-urlencoded or multipart/form-data bodies.
• Context.FormFile(key string) (*multipart.FileHeader, error) — returns an uploaded file from a multipart form.

Motivation

Form handling is a common web pattern for HTML forms, file uploads, and API endpoints. Currently users must parse the body manually. All major frameworks provide these helpers.

Scope

• Parse Content-Type header to determine form type.
• For application/x-www-form-urlencoded: use url.ParseQuery on body.
• For multipart/form-data: use mime/multipart package.
• Cache parsed form data on the Context (similar to query caching).
• Consider memory limits for multipart uploads (configurable max body size).

Non-breaking

Additive methods on Context. No existing API changes required.

[FumingPower3925]

Implemented in v1.0.0. Closing as part of final review.