refactor: parse item_fields into Pydantic models in all webhook handlers

[gillespied]

Context

In #54, the new handle_qa_review handler parses raw SharePoint item_fields dicts into a typed Pydantic model (SharepointInvitationQA) as its first step, using from_sharepoint_fields(). This gives us:

• Pydantic validation up front (catches malformed data early)
• SharePoint system fields (Modified, Created, AuthorLookupId, etc.) stripped automatically
• Typed attribute access instead of .get() calls with default strings
• Consistent serialisation when writing back via to_sharepoint_fields()

The existing make_list_review_handler (handle_review) and extract_actions_from_review still work with raw item_fields dicts throughout. This means:

• No validation of incoming SharePoint data
• Manual .get() calls with hardcoded defaults scattered across handler and component code
• Risk of passing system fields through to downstream consumers

Proposal

Adopt the same pattern for the existing handlers:

1. handle_review in route_handlers.py — parse item_fields into the appropriate SharePoint model (SharepointInvitation or SharepointSubmission depending on document_type) as the first step
2. extract_actions_from_review in components.py — accept a typed model instead of a raw dict, build context strings from model attributes rather than .get() calls
3. to_sharepoint_action in mappers.py — accept a typed source model instead of item_fields: dict

This would make from_sharepoint_fields() the standard entry point for all SharePoint data entering the handler pipeline.

Files affected

• src/box2/receiver/route_handlers.py — handle_review inner function
• src/box2/pipeline/components.py — extract_actions_from_review(), context template formatting
• src/box2/pipeline/mappers.py — to_sharepoint_action() signature

Notes

• This is a refactor with no functional change — existing behaviour should be preserved
• The from_sharepoint_fields utility and round-trip tests already exist from feat: add private office QA stage for invitations #54
• The QA handler serves as the reference implementation for the pattern