Part 2 - Tool Deep Linking: request validation, content items, and signed responses

[eliknebel]

This PR adds complete tool-side Deep Linking support on top of the core refactor. It validates LtiDeepLinkingRequest
launches, parses typed deep-linking settings, builds validated content items, and generates signed deep-linking
response JWTs.

This PR should be reviewed and landed after #42

What Changed

• Added tool deep-linking public APIs:
  • Lti_1p3.Tool.validate_deep_linking_request/1
  • Lti_1p3.Tool.deep_linking_content_item/2
  • Lti_1p3.Tool.build_deep_linking_response/3
• Added deep-linking modules for:
  • typed request/settings parsing
  • request validation
  • content item building
  • response building
  • JWT signing
  • telemetry
  • compatibility policy hooks
• Added shared deep-linking claim key helper:
  • Lti_1p3.DeepLinking.ClaimKeys
• Updated message dispatch to treat deep-linking launches as a first-class validated path
• Added deep-linking guide, QA report, and requirement matrix docs

Why

The core PR establishes launch correctness and message dispatch, but deep-linking flows still need request validation
and response generation. This PR closes that gap with a typed, explicit API that is ready for real LMS
interoperability work.

Reviewer Notes

Focus review on:

• deep-linking request/settings parsing
• content item validation
• JWT response claim construction/signing
• deterministic error semantics
• LMS compatibility hooks

Most of the diff against master is inherited from PR 1. The incremental review surface here is the new
Tool.DeepLinking modules and the dispatch wiring.

Documentation

• docs/tool_deep_linking_guide.md
• docs/features/tool-deep-linking/*
• README.md
• CHANGELOG.md