Fix build break on Windows runner

[Branden Bonaby (bbonaby)]

📖 Description

• Remove the obsolete builtin_test_server field from the Windows PSEC proxy test helper's ProxyConfig initializer.
• Restore Windows test compilation and Clippy without changing runtime behavior.

🔗 References

• Fixes the main-branch build failure observed in the Windows checks on Document supported sandbox policy contracts #1416. That PR is unchanged.

🔍 Validation

Commands run from src/:

• cargo check -p mxc-sdk --tests --locked — reproduced E0560 before the fix; passed afterward.
• cargo fmt --all -- --check — passed.
• cargo clippy -p mxc-sdk --all-targets --locked -- -D warnings — passed.
• cargo test -p mxc-sdk --lib --locked psec_1_0 -- --test-threads=1 — passed, 7 tests.

No real sandbox workloads were run. Windows ARM64 and the full release/all-features CI matrix were not run locally.

✅ 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 pull request builds)

📋 Issue Type

• Bug fix
• Feature
• Task

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Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Copilot review overview

🟢 Approval recommended

The test-only removal matches the current struct definition, with no unresolved issues.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes Windows proxy test compilation by aligning the test helper with the current ProxyConfig definition.

Changes:

• Removes the obsolete builtin_test_server initializer field without changing runtime behavior.

File	Description
src/​mxc-sdk/​src/​backends/​process_container/​common/​base_container_runner.rs	Removes an obsolete field from the proxy test helper.

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