MessagePart cannot represent source attribution, so every provider drops citations

MessagePart (src/Messages/DTO/MessagePart.php) can carry exactly seven things:
channel, type, thoughtSignature, text, file, functionCall,
functionResponse. None of them can hold a citation.

All three first-party providers return source attribution, in three different
shapes (per vendor documentation):

And all three provider packages currently drop it. Verified by reading the source:
AnthropicTextGenerationModel::parseResponseContentMessagePart()'s case 'text'
keeps only $partData['text']; GoogleTextGenerationModel and
OpenAiTextGenerationModel contain no citation/grounding/annotation handling at all.

This is a gap rather than a data-loss bug — no provider currently requests
citations, so nothing is being lost in production today. But the parse paths are
not ready, and no provider can be made ready while the DTO has nowhere to put the
result.

Suggested shape: thoughtSignature is the precedent — an opaque,
provider-supplied, round-trippable field already accepted on MessagePart. A
citation carrier could follow it, normalising the three vendor shapes into one
representation (source identifier/URI and optional title).

Related: #268 (provider-agnostic text extraction) — same theme of getting
structured meaning out of documents.

Happy to open a PR once maintainers settle the shape; the API-surface decision is
yours.

Start with src/Messages/DTO/MessagePart.php, then inspect AnthropicTextGenerationModel::parseResponseContentMessagePart(), GoogleTextGenerationModel, and OpenAiTextGenerationModel. First confirm the provider-specific citation shapes described in the issue; implementation is blocked until maintainers decide the normalized carrier shape. Done means the DTO and all three parse paths can preserve citations, with coverage for each provider shape.