phoenix-wasm: add Conn.to_raw_response/1 for HTTP response serialization

[yevbar]

By Sneezy

Summary

Adds Conn.to_raw_response/1 — the response-side counterpart of to_raw_request/1 — completing the full HTTP request/response lifecycle for the WASM pipeline:

Raw HTTP request → Conn (parse) → middleware/routing → Conn → Raw HTTP response

Changes

Conn.to_raw_response/1

Converts a Conn's response state (status, resp_headers, resp_body) into a complete HTTP/1.1 response string:

conn = Conn.new("GET", "/api/users")
|> Conn.json(200, %{users: []})

Conn.to_raw_response(conn)
# => "HTTP/1.1 200 OK\r\ncontent-type: application/json; charset=utf-8\r\ncontent-length: 12\r\n\r\n{\"users\":[]}"

• Auto-adds Content-Length header when not already set
• Uses IO lists internally for zero intermediate string allocations (~1.5µs/op)
• Handles all response types: JSON, HTML, text, redirects, error pages

Conn.status_reason/1

Maps HTTP status codes to standard reason phrases:

Conn.status_reason(200)  # => "OK"
Conn.status_reason(404)  # => "Not Found"
Conn.status_reason(302)  # => "Found"

Covers all common HTTP/1.1 status codes (1xx through 5xx).

Tests

23 new tests covering:

• All common status codes (200, 201, 204, 301, 302, 303, 404, 500)
• Content types (JSON, HTML, text/plain)
• Redirect responses with location headers
• Content-Length auto-generation and override behavior
• Edge cases (nil status, nil body, unknown status codes)
• Round-trip parsing (response → parse headers/body → verify)
• Full request+response lifecycle integration
• Performance benchmark (~1.5µs/op)