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/ingestion/reader.py

@@ -1,3 +1,18 @@
+"""
+High-level mail ingestion orchestration for Mail Intake.
+
+This module provides the primary, provider-agnostic entry point for
+reading and processing mail data.
+
+It coordinates:
+- Mail adapter access
+- Message and thread iteration
+- Header and body parsing
+- Normalization and model construction
+
+No provider-specific logic or API semantics are permitted in this layer.
+"""
+
 from datetime import datetime
 from typing import Iterator, Dict, Any
 
@@ -14,22 +29,43 @@ class MailIntakeReader:
     """
     High-level read-only ingestion interface.
 
-    This is the primary entry point users should interact with.
-    It orchestrates:
-    - adapter calls
-    - parsing
-    - normalization
-    - model construction
+    This class is the **primary entry point** for consumers of the Mail
+    Intake library.
 
-    No provider-specific logic exists here.
+    It orchestrates the full ingestion pipeline:
+    - Querying the adapter for message references
+    - Fetching raw provider messages
+    - Parsing and normalizing message data
+    - Constructing domain models
+
+    This class is intentionally:
+    - Provider-agnostic
+    - Stateless beyond iteration scope
+    - Read-only
     """
 
     def __init__(self, adapter: MailIntakeAdapter):
+        """
+        Initialize the mail reader.
+
+        Args:
+            adapter: Mail adapter implementation used to retrieve raw
+                messages and threads from a mail provider.
+        """
         self._adapter = adapter
 
     def iter_messages(self, query: str) -> Iterator[MailIntakeMessage]:
         """
         Iterate over parsed messages matching a provider query.
+
+        Args:
+            query: Provider-specific query string used to filter messages.
+
+        Yields:
+            Fully parsed and normalized `MailIntakeMessage` instances.
+
+        Raises:
+            MailIntakeParsingError: If a message cannot be parsed.
         """
         for ref in self._adapter.iter_message_refs(query):
             raw = self._adapter.fetch_message(ref["message_id"])
@@ -39,7 +75,17 @@ class MailIntakeReader:
         """
         Iterate over threads constructed from messages matching a query.
 
-        Messages are grouped by thread_id and yielded as complete threads.
+        Messages are grouped by `thread_id` and yielded as complete thread
+        objects containing all associated messages.
+
+        Args:
+            query: Provider-specific query string used to filter messages.
+
+        Returns:
+            An iterator of `MailIntakeThread` instances.
+
+        Raises:
+            MailIntakeParsingError: If a message cannot be parsed.
         """
         threads: Dict[str, MailIntakeThread] = {}
 
@@ -61,7 +107,17 @@ class MailIntakeReader:
 
     def _parse_message(self, raw_message: Dict[str, Any]) -> MailIntakeMessage:
         """
-        Parse a raw provider message into a MailIntakeMessage.
+        Parse a raw provider message into a `MailIntakeMessage`.
+
+        Args:
+            raw_message: Provider-native message payload.
+
+        Returns:
+            A fully populated `MailIntakeMessage` instance.
+
+        Raises:
+            MailIntakeParsingError: If the message payload is missing required
+                fields or cannot be parsed.
         """
         try:
             message_id = raw_message["id"]