Python: Discard unsafe Content fields during deepcopy

[Sweetteabittersugar]

Motivation & Context

Content.__deepcopy__ currently keeps fields listed in _SHALLOW_COPY_FIELDS by reference. This creates a partially deep-copied object that can still share mutable provider SDK state with the original. The issue proposes discarding designated unsafe data and logging the loss so deepcopy retains isolation without failing on opaque provider objects.

Description & Review Guide

• What are the major changes?
  • Set every non-None Content._SHALLOW_COPY_FIELDS value to None in the clone and emit a warning.
  • Continue deep-copying all other Content fields normally.
  • Update direct deepcopy, response aggregation, and text-coalescing tests for the new contract.
• What is the impact of these changes?
  • Deep-copied Content objects no longer share provider-specific raw state with their source.
  • Callers that inspect raw_representation after a Content deepcopy will receive None; the original object remains unchanged.
  • SerializationMixin behavior is intentionally unchanged because Python: [Bug]: Should Content.__deepcopy__ preserve unsafe fields #7851 is scoped to Content.
• What do you want reviewers to focus on?
  • Whether fields explicitly classified in _SHALLOW_COPY_FIELDS should always be discarded, rather than copied on a best-effort basis.

Related Issue

Fixes #7851

This is an alternative to #7898. That PR first attempts to deepcopy marked values and discards them only on failure, and it also changes SerializationMixin. This PR applies the issue's narrower Content contract directly: every non-None designated unsafe value is discarded. Besides avoiding copies of values already classified as unsafe, this approach cannot leave partial entries in the shared deepcopy memo after a failed best-effort copy.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Updates Content.__deepcopy__ to discard unsafe shallow-copy fields while preserving isolation.

Changes:

• Replace unsafe fields with None and log warnings.
• Update deepcopy and response-coalescing tests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
python/packages/core/agent_framework/_types.py	Implements safe field discarding during deepcopy.
python/packages/core/tests/core/test_types.py	Updates tests for the new behavior.

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[Sweetteabittersugar]

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Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_types.py	1302	98	92%	63, 73, 127, 136, 155, 157, 161, 165, 167, 169, 171, 189, 193, 219, 241, 246, 251, 255, 285, 713–714, 895–896, 1354, 1429, 1467, 1487, 1497, 1691–1693, 1978–1983, 2008, 2063, 2068, 2078, 2086, 2093–2097, 2115, 2188, 2201, 2206, 2319, 2603, 2627, 2726, 2907–2908, 3010, 3293, 3346, 3365, 3404, 3415, 3417–3421, 3423, 3426–3434, 3444, 3609, 3616, 3620–3622, 3767, 3772, 3777, 3782, 3786, 3872–3874, 3903, 4019–4023
TOTAL	48229	4483	90%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
9764	36 💤	0 ❌	0 🔥	2m 37s ⏱️