Centralize Content-Type handling with detailed error messages

[Copilot]

Addresses requirements to consolidate Content-Type parsing, improve error messages, and support custom validation.

Changes

New module: http/content_type.rs

• parse_content_type() - centralized parser with detailed error hints
• validate_content_type() - enforce specific type/subtype requirements
• is_multipart_form_data() - typed helper for common check
• ContentTypeError - structured error type with specific variants

Simplified existing code

• ops/mod.rs: extract_mime() now delegates to centralized parser (12 → 3 lines)
• ops/signature.rs: uses is_multipart_form_data() helper
• http/de.rs: added parse_field_value_content_type() for consistency

Error message improvements

Before:

invalid content type: "invalid"

After:

Invalid Content-Type header: "invalid". 
Content-Type should be in format 'type/subtype' (e.g., 'text/plain', 'application/json')

Errors now include:

• Detected issues (missing type/subtype, malformed syntax, etc.)
• Expected format with examples
• Multiple hints when applicable

Custom validation example

use s3s::http::validate_content_type;

let mime: Mime = content_type.parse()?;
validate_content_type(&mime, "application", "json")?;
// Returns descriptive error if type doesn't match

---

Related to issue requirements for centralized Content-Type handling.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Content-Type handling</issue_title>
<issue_description>+ [ ] Extract all content type handling to one place

• Return detailed error message about invalid content type
• Support custom content type (?)
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Content-Type handling #385

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.