Make subprocess stderr routing an explicit policy (#340)

[leynos]

Summary

Closes #340

Replaces the suppress_stderr boolean with the named policy type the issue proposes:

pub enum StderrMode {
    Forward,
    Suppress,
}

• The runner selects the policy from CLI state in ninja_process_options (the only place that consults Cli::resolved_diag_json(); the process layer stopped calling it in Decouple runner process execution from Cli (#339) #371).
• NinjaProcessOptions, run_command_and_stream, and spawn_and_stream_output consume the policy.
• Under Suppress, child stderr is still drained but discarded, so JSON diagnostics keep stderr machine-readable exactly as before.

Stacked on #371 (designed together per #347); retargets to main automatically once #371 merges.

Validation

• make check-fmt / make lint / make test — pass (37 suites; JSON-diagnostics BDD scenarios unchanged and green)

🤖 Generated with Claude Code

Summary by Sourcery

Introduce an explicit stderr routing policy for subprocesses and wire it through the runner and process layers.

Enhancements:

• Replace the boolean stderr suppression flag with a StderrMode enum to make subprocess stderr handling explicit and self-describing.
• Propagate the new stderr routing policy through NinjaProcessOptions and subprocess execution helpers so JSON diagnostics can suppress child stderr while still draining it.

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[sourcery-ai]

Reviewer's Guide

Replace the boolean stderr suppression flag with an explicit StderrMode policy and thread it through the runner and process layers so stderr routing is decided at the runner boundary while preserving existing JSON-diagnostics behavior.

Sequence diagram for stderr_mode selection and routing

sequenceDiagram
    actor User
    participant Cli
    participant Runner
    participant Process

    User->>Cli: set resolved_diag_json()
    Runner->>Runner: ninja_process_options(cli)
    Runner->>Runner: set stderr_mode (Forward or Suppress)
    Runner->>Process: run_ninja_build_internal(request.options.stderr_mode)
    Process->>Process: run_command_and_stream(cmd, status_observer, stderr_mode)
    Process->>Process: spawn_and_stream_output(child, status_observer, stderr_mode)
    alt [stderr_mode.is_suppressed()]
        Process->>Process: forward_child_output(stderr, io::sink(), stderr)
    else [not suppressed]
        Process->>Process: forward_child_output(stderr, io::stderr(), stderr)
    end

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File-Level Changes

Change	Details	Files
Introduce StderrMode enum and replace suppress_stderr boolean throughout the subprocess pipeline.	Add StderrMode enum with Forward and Suppress variants plus helper is_suppressed() to express stderr routing policy. Change NinjaProcessOptions to hold stderr_mode instead of a suppress_stderr boolean. Update run_command_and_stream and spawn_and_stream_output signatures and internal logic to consume StderrMode and use is_suppressed() when deciding whether to forward or discard stderr.	src/runner/process/mod.rs
Select stderr routing policy in the runner based on CLI JSON diagnostics mode and pass it into process options.	Derive stderr_mode in ninja_process_options from Cli::resolved_diag_json(), mapping JSON mode to Suppress and non-JSON mode to Forward. Propagate stderr_mode through run_ninja_build_internal and run_ninja_tool_internal calls instead of the old suppress_stderr flag.	src/runner/mod.rs

Possibly linked issues

• Make subprocess stderr routing an explicit policy #340: PR replaces suppress_stderr with StderrMode, moves policy choice to runner, and preserves JSON stderr behavior.

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