Issue #3586448: Add ChatWithPdf capability to AiModelCapability enum

Description

Add ChatWithPdf = 'chat_with_pdf' to the AiModelCapability enum so provider plugins can advertise PDF-in-chat support and consumers can require it.

The new case is wired into all three match arms:

• getBaseOperationType() → 'chat'
• getTitle() → 'Chat with PDF'
• getDescription() → 'Is set if the chat model can include PDF documents in the chat input.'

Scope: enum addition only. Provider plugins (Anthropic, Gemini, OpenAI) that support PDF input can opt in to the new capability separately.

Testing instructions

1. Enable the ai module on a Drupal site.
2. In a provider plugin's getSupportedCapabilities() method, return AiModelCapability::ChatWithPdf.
3. Verify AiModelCapability::ChatWithPdf->getBaseOperationType() returns 'chat', getTitle() returns 'Chat with PDF', and getDescription() returns the expected string.
4. Alternatively, run: php -r "require 'vendor/autoload.php'; var_dump(\Drupal\ai\Enum\AiModelCapability::ChatWithPdf->getTitle());"

Checklist

• I have linked the related issue in the MR title or description
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

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Closes #3586448 (closed)