Remove remaining pre-v0.9 runtime vestiges

[Gudge (MGudgin)]

📖 Description

This PR removes the remaining pre-v0.9 runtime vestiges after the backend cleanup in the preceding stack PRs. It drops obsolete shared network fields, the built-in test proxy, retired CLI testing switches, and the unused Unix proxy coordinator while preserving supported directional and caller-managed proxy behavior. It also redacts non-string proxy diagnostics and clarifies accepted WSLC network postures.

Details

• Remove unrepresentable shared runtime fields and obsolete testing plumbing.
• Simplify ProcessContainer audit and proxy ownership without weakening enforcement.
• Update schema, telemetry, and backend guidance for supported requests.
• Redact non-string proxy diagnostics and clarify supported networking guidance.

🔗 References

Depends on #1424; the stack begins at #1421. #1397 is the single-commit alternative to these five PRs; merge either the stack or the combined PR, not both. Follows #1382 and #1383.

🔍 Validation

Tests

• From src/, cargo fmt --all -- --check: passed.
• cargo check --workspace --all-targets --all-features --quiet: passed.
• cargo clippy --workspace --all-targets --all-features --quiet -- -D warnings: passed.
• cargo test --workspace --quiet: passed.
• cargo test -p mxc-sdk --lib --features wslc,isolation_session,microvm,hyperlight --quiet: 3,293 passed, 3 ignored.
• cargo test -p mxc-sdk --lib non_string_proxy_values_in_comments_are_redacted --quiet: 1 passed on Windows.
• cargo check -p mxc-sdk --all-targets --target x86_64-apple-darwin --quiet: passed.
• Debian WSL, from src/, cargo check -p mxc-sdk --all-targets --quiet: passed.
• Debian WSL, cargo test -p mxc-sdk --lib lxc::common --quiet -- --test-threads=1: 302 passed.
• Debian WSL, cargo test -p mxc-sdk --lib bubblewrap::common --quiet -- --test-threads=1: 301 passed.
• Debian WSL, cargo build -p lxc -p unix_test_proxy --quiet: passed; bash ../tests/scripts/run_bwrap_directional_test.sh: nine checks passed.
• node --test scripts/versioning/tests/check-contract-codegen.test.js: 9 passed; node scripts/versioning/check-contract-codegen.js --check: passed, three artifact sets matched.
• Node/.NET SDK suites and native macOS/LXC runtime tests 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 docs/pull-requests.md)

📋 Issue Type

• Bug fix
• Feature
• Task

---

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[Gudge (MGudgin)]

Superseded by merged #1397, which includes the final shared-model and proxy cleanup in this stacked PR. Closing this alternative PR.