fix(citations): Preserve tagged union structure for CitationLocation

[danilop]

Description

This PR fixes citation location handling in the Bedrock Converse API to properly preserve the tagged union structure required by the API.

Problem:
The Bedrock API requires CitationLocation to be a tagged union with exactly one wrapper key (documentChar, documentPage, or documentChunk). The previous implementation was flattening the structure, causing validation errors:

botocore.exceptions.ParamValidationError: Must set one of the following keys 
for tagged union structure: web, documentChar, documentPage, documentChunk, searchResultLocation

Solution:

• Refactored citation type definitions to use *Inner types + wrapper types for proper tagged union modeling
• Added _format_citation_location helper method to preserve the union structure when filtering
• Added comprehensive tests for all document-based citation location types

BREAKING CHANGE: CitationLocation types now use wrapper structure (e.g., {"documentChar": {"documentIndex": 0, ...}}) instead of flat structure. This matches the actual Bedrock API specification and fixes the runtime error.

Related Issues

Fixes #1323

Type of Change

Bug fix (breaking - but the previous behavior was incorrect and caused runtime errors)

Testing

• I ran hatch run prepare (1615 tests pass)
• Tests for documentChar, documentPage, documentChunk location types
• Tests for field filtering and unknown location types

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.