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/exceptions.py

@@ -1,19 +1,49 @@
+"""
+Exception hierarchy for Mail Intake.
+
+This module defines the **canonical exception types** used throughout the
+Mail Intake library.
+
+All library-raised errors derive from `MailIntakeError`. Consumers are
+encouraged to catch this base type (or specific subclasses) rather than
+provider-specific or third-party exceptions.
+"""
+
+
 class MailIntakeError(Exception):
     """
-    Base exception for all mail-intake errors.
+    Base exception for all Mail Intake errors.
 
-    Users of the library should catch this type (or subclasses)
-    instead of provider-specific or third-party exceptions.
+    This is the root of the Mail Intake exception hierarchy.
+    All errors raised by the library must derive from this class.
+
+    Consumers should generally catch this type when handling
+    library-level failures.
     """
 
 
 class MailIntakeAuthError(MailIntakeError):
-    """Authentication and credential-related failures."""
+    """
+    Authentication and credential-related failures.
+
+    Raised when authentication providers are unable to acquire,
+    refresh, or persist valid credentials.
+    """
 
 
 class MailIntakeAdapterError(MailIntakeError):
-    """Errors raised by mail provider adapters."""
+    """
+    Errors raised by mail provider adapters.
+
+    Raised when a provider adapter encounters API errors,
+    transport failures, or invalid provider responses.
+    """
 
 
 class MailIntakeParsingError(MailIntakeError):
-    """Errors encountered while parsing message content."""
+    """
+    Errors encountered while parsing message content.
+
+    Raised when raw provider payloads cannot be interpreted
+    or normalized into internal domain models.
+    """