Prove observable MCP required optional contracts
Added CLI JSON regression coverage for MCP required versus optional flags, redacted env/header values, degraded malformed config reporting, and failed unsupported usage reporting without touching runtime internals.
Constraint: Task 12 scope preferred rusty-claude-cli tests and avoid worker-1/3 MCP internals.
Rejected: Runtime lifecycle edits | existing observable JSON contracts already expose required, redacted keys, degraded config, and unsupported-action failure semantics.
Confidence: high
Scope-risk: narrow
Directive: Preserve secret-value redaction by exposing env/header keys only; keep degraded config distinct from usage errors.
Tested: cargo fmt --manifest-path Cargo.toml -p rusty-claude-cli --check; cargo test --manifest-path Cargo.toml -p rusty-claude-cli --test output_format_contract mcp_ -- --nocapture; cargo check --manifest-path Cargo.toml -p rusty-claude-cli.
Not-tested: Full output_format_contract currently has unrelated pre-existing failures in plugin/doctor contract tests.
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