.NET: Prevent telemetry serialization failures from failing workflows

[Peter Ibekwe (peibekwe)]

Description & Review Guide

Workflow telemetry now safely falls back to an [Unserializable: ...]  representation when sensitive payload serialization fails, without disrupting execution or altering the message stream. Adds regression coverage for unsupported AIContent, executor telemetry, AOT, and arbitrary serialization exceptions.

CodeInterpreterToolCallContent that was mentioned in the issue is now supported by current MEAI serialization, but the underlying telemetry failure remained for unsupported or custom content types. This change safely falls back to [Unserializable: ...] without failing workflow execution or altering the message stream.

Fixes #5066

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

Adds resilient workflow telemetry serialization so unsupported payloads fall back without disrupting execution.

Changes:

• Catches serialization failures and emits an [Unserializable: ...] fallback.
• Adds regression coverage for message delivery, executor telemetry, AOT-like failures, and custom exceptions.

Reviewed changes

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

File	Description
WorkflowTelemetryContext.cs	Adds telemetry serialization fallback.
ObservabilityTests.cs	Tests fallback behavior and workflow continuity.

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[github-actions]

Agent Framework Review — Iteration 1

Completed passes: 5 | Result: No high-severity findings

Scope: full PR (1 commit(s)): 7a45235d304e

Review passes

• Correctness (gpt-5.6-sol) — No issues found in this pass.
• Security Reliability (claude-opus-4.8) — No issues found in this pass.
• Test Coverage (gpt-5.6-sol) — No issues found in this pass.
• Failure Modes (claude-opus-4.8) — No issues found in this pass.
• Design Approach (claude-opus-4.8) — No issues found in this pass.

[github-actions]

Agent Framework Review — Iteration 2

Completed passes: 5 | Result: No high-severity findings

Scope: 1 net-new commit(s): 1ca94adcb18d

Review passes

• Correctness (gpt-5.6-sol) — No issues found in this pass.
• Security Reliability (claude-opus-4.8) — No issues found in this pass.
• Test Coverage (gpt-5.6-sol) — No issues found in this pass.
• Failure Modes (claude-opus-4.8) — No issues found in this pass.
• Design Approach (claude-opus-4.8) — No issues found in this pass.