docs(mail_intake): add comprehensive docstrings across ingestion, adapters, auth, and parsing layers

- docs(mail_intake/__init__.py): document module-based public API and usage patterns
- docs(mail_intake/ingestion/reader.py): document high-level ingestion orchestration
- docs(mail_intake/adapters/base.py): document adapter contract for mail providers
- docs(mail_intake/adapters/gmail.py): document Gmail adapter implementation and constraints
- docs(mail_intake/auth/base.py): document authentication provider contract
- docs(mail_intake/auth/google.py): document Google OAuth authentication provider
- docs(mail_intake/models/message.py): document canonical email message model
- docs(mail_intake/models/thread.py): document canonical email thread model
- docs(mail_intake/parsers/body.py): document message body extraction logic
- docs(mail_intake/parsers/headers.py): document message header normalization utilities
- docs(mail_intake/parsers/subject.py): document subject normalization utilities
- docs(mail_intake/config.py): document global configuration model
- docs(mail_intake/exceptions.py): document library exception hierarchy

mail_intake/models/message.py

@@ -1,3 +1,14 @@
+"""
+Message domain models for Mail Intake.
+
+This module defines the **canonical, provider-agnostic representation**
+of an individual email message as used internally by the Mail Intake
+ingestion pipeline.
+
+Models in this module are safe to persist and must not contain any
+provider-specific fields or semantics.
+"""
+
 from dataclasses import dataclass
 from datetime import datetime
 from typing import Optional, Dict
@@ -8,19 +19,37 @@ class MailIntakeMessage:
     """
     Canonical internal representation of a single email message.
 
-    This model is provider-agnostic and safe to persist.
-    No Gmail-specific fields should appear here.
+    This model represents a fully parsed and normalized email message.
+    It is intentionally provider-agnostic and suitable for persistence,
+    indexing, and downstream processing.
+
+    No provider-specific identifiers, payloads, or API semantics
+    should appear in this model.
     """
 
     message_id: str
+    """Provider-specific message identifier."""
+
     thread_id: str
+    """Provider-specific thread identifier to which this message belongs."""
+
     timestamp: datetime
+    """Message timestamp as a timezone-naive UTC datetime."""
 
     from_email: str
+    """Sender email address."""
+
     from_name: Optional[str]
+    """Optional human-readable sender name."""
 
     subject: str
+    """Raw subject line of the message."""
+
     body_text: str
+    """Extracted plain-text body content of the message."""
+
     snippet: str
+    """Short provider-supplied preview snippet of the message."""
 
     raw_headers: Dict[str, str]
+    """Normalized mapping of message headers (header name → value)."""