Add Citation DTO core contract to MessagePart

[the-hercules]

Implements #277 (Sets up core DTO foundation).
This PR establishes the core DTO foundation to support provider-agnostic source citations, addressing the API-
surface blocker discussed in #277.

Following the feedback and consensus in the issue, this introduces a normalized Citation object and attaches it
to MessagePart. The DTO is shaped to represent the common citation formats from Google, OpenAI, and Anthropic
while keeping the sources checkable:

• Typed Source Identity: Replaces a generic URI with explicit, nullable url (for remote sources) and
  documentIndex (for request-provided documents) to avoid string sniffing.
• Text Offset Ranges: Includes startIndex and endIndex to anchor the citation locally to the owning
  MessagePart's text string.
  • Quoted Text: Includes quotedText to cleanly carry Anthropic's block-based cited_text.
  • Validation: Enforces that offset ranges are strictly non-negative and properly ordered (startIndex <= endIndex), throwing an InvalidArgumentException otherwise.

Note: This PR handles the core contract/DTOs within the agnostic client. Follow-up PRs in the respective provider plugin repositories (ai-provider-for-anthropic, etc.) will be required to update their parsing logic to utilize this new format.

AI Disclosure
Model - Gemini 3.1 Pro
Usage - Implementation of functionality and test after discussing in Plan Mode
Human Loop - Reviewed code against the issue's requirements

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[codecov]

Codecov Report

❌ Patch coverage is 98.42520% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.94%. Comparing base (0a9a1b3) to head (6690dc0).

Files with missing lines	Patch %	Lines
src/Messages/DTO/MessagePart.php	94.87%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #283      +/-   ##
============================================
+ Coverage     86.60%   86.94%   +0.34%     
- Complexity     1335     1362      +27     
============================================
  Files            68       69       +1     
  Lines          4314     4436     +122     
============================================
+ Hits           3736     3857     +121     
- Misses          578      579       +1     

Flag	Coverage Δ
unit	86.94% <98.42%> (+0.34%)	⬆️

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[zahidgh]

One validation gap: documentIndex represents an index into the request’s documents array, but the constructor and JSON schema currently allow negative values. A Citation(null, -1, ...) can therefore serialize successfully and leave providers to handle an invalid reference. Please reject documentIndex < 0, add minimum: 0 to its schema, and cover the guard with a unit test.