[Content Understanding] Update toLlmInput page markers and filter LLMStats telemetry

[chienyuanchang]

Description

Updates the azure-ai-contentunderstanding LlmInputHelper.toLlmInput() helper to align its rendered output with the upcoming service page-marker format and to remove non-user-facing telemetry from RAI warning output.

Changes made:

• Updated SDK-injected document page markers from <!-- page N --> to <!-- InputPageNumber: N -->.
• Added duplicate-marker defense: if service markdown already contains <!-- InputPageNumber:, toLlmInput() does not inject additional page markers.
• Filtered service-emitted internal telemetry warnings whose message starts with LLMStats: from the rendered rai_warnings front matter.
• Preserved LLMStats: text when it appears in the document markdown body; only structured warnings are filtered.
• Updated unit tests and sample tests for the new marker format and warning-filter behavior.
• Updated CHANGELOG.md.

Relevant issues / context:

• Design proposal: https://github.com/cognitive-services/ContentUnderstanding-Docs/issues/249
• Agent Framework feedback that prompted the LLMStats: filtering: Python: Adopt azure-ai-contentunderstanding to_llm_input in CU context provider microsoft/agent-framework#5796

Companion PRs (sibling SDKs):

• Python: [Content Understanding] Update to_llm_input page markers and filter telemetry warnings azure-sdk-for-python#47326
• .NET: [Content Understanding] Update ToLlmInput page markers and filter LLMStats telemetry azure-sdk-for-net#59724
• JS: https://github.com/Azure/azure-sdk-for-js/pull/new/cu-sdk/llm-input-helper-update

This PR is not based on regenerated SDK code from a new swagger / TypeSpec spec.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
  • No public API signatures are changed. This only changes rendered text produced by the preview LlmInputHelper.toLlmInput() helper.
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message.

Testing Guidelines

• Pull request includes test coverage for the included changes (31 unit tests pass).