[Isolation Session] State aware tests now correctly check stderr for malformed_id

[Elliot (theelliotm)]

📖 Description

The malformed_id state aware test went stale during PR #1308, which deliberately moved non-dry-run exec error envelopes from stdout to stderr. This PR makes the test now correctly check stderr for malformed_id during actual execution, while still checking stdout during other phases.

🔗 References

Resolves #1324

🔍 Validation

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)
• If this PR changes Cargo.lock, the dependency-feed-check check passes (see docs/pull-requests.md)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused test fix matches the documented executor behavior and preserves existing handling for all other phases.

Review effort: Balanced
Findings: None

What changed in this PR

Updates the IsolationSession state-aware test to follow the phase-aware error stream contract.

Changes:

• Parses real exec failure envelopes from stderr.
• Continues parsing other phases and dry-run failures from stdout.

File	Description
tests/​scripts/​run_isolation_session_state_aware_tests.ps1	Corrects malformed_id envelope parsing for real exec requests.

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[adpa-ms]

[AgentAssisted] Validated this fix on a test machine with the IsolationSession service (x64), running tests/scripts/run_isolation_session_state_aware_tests.ps1 against a wxc-exec built from main at b6378934 (this PR's base) with the isolation_session feature:

• Without the fix: 69/70 passed. The only failure was exec (real): error.code is 'malformed_id' (got '<no envelope>'), matching the nightly.
• With a logically identical change (the same condition on one line): 70/70 passed, and that assertion reports (got 'malformed_id').

The Scheduled Validation Tests workflow, limited to the isolation-session jobs and run on a branch with that change, passed on x64 and arm64, including state-aware 70/70: https://github.com/microsoft/mxc/actions/runs/36610370509