Added a 256kb custom properties truncation limit on specific GenAI attributes

[rads-1996]

Description

The following GenAI attributes have a truncation limit of 256kb. All other custom properties will be truncated at 64kb as before -

- gen_ai.input.messages
- gen_ai.output.messages
- gen_ai.system_instructions
- gen_ai.tool.definitions
- gen_ai.tool.call.arguments
- gen_ai.tool.call.result
- gen_ai.evaluation.explanation

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• 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. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

Updates azure-monitor-opentelemetry-exporter custom property filtering so a specific allowlist of GenAI attributes is truncated at a higher limit (256 * 1024) while all other custom properties continue to truncate at the existing 64 * 1024 limit.

Changes:

• Increase truncation limit for _GEN_AI_ATTRIBUTES values to 256 * 1024.
• Update and extend unit tests to cover mixed attributes and the new 256KB/KiB truncation behavior.
• Add a corresponding unreleased changelog entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_utils.py	Applies a higher truncation cap for _GEN_AI_ATTRIBUTES in _filter_custom_properties.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_utils.py	Updates existing truncation tests and adds explicit coverage for GenAI truncation at 256 * 1024.
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Documents the new GenAI-specific truncation limit under unreleased features.

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