Factor out shared header-building logic in send APIs

[MatthewCane]

Rationale

send and send_file build similar headers; deduplicating reduces drift and centralizes formatting logic.

Files

• python_ntfy/_send_functions.py (or a new internal helper module)

Tasks

• Introduce a private helper (e.g., _build_headers(...)) to construct request headers
• Use the helper from both send and send_file
• Keep behavior backward-compatible (same headers for same inputs)

Acceptance criteria

• Headers generated for equivalent inputs are identical to current behavior (except any explicitly approved changes)
• Tests pass with coverage unchanged or improved