Keep interactive stdin runtime alive after turn failures

[njfio]

Problem statement

When interactive runtime is running in stdin mode (used by tau-tui subprocess orchestration), a single turn failure currently bubbles as an error and terminates the process with exit status 1.

This causes Tau TUI to exit entirely on provider/turn errors such as:

• invalid response: codex cli failed with status 1
• warning: no last agent message

TTY mode already reports turn errors and continues. Stdin mode should match this non-fatal behavior for turn-level failures.

Acceptance criteria

1. Turn-level failures during interactive stdin mode do not terminate the runtime process.
2. Failure is still surfaced via existing error reporting (stderr + turn.failed event).
3. Runtime continues accepting subsequent prompts after a failed turn.
4. Regression test(s) cover non-fatal behavior for turn failure handling path.

Non-goals

• Changing hard-fail behavior for startup/configuration errors.
• Altering provider/tool error semantics.
• Redesigning TUI layout/output formatting.

[njfio]

Spec added: specs/3573-stdin-turn-failure-nonfatal.md\n\nProceeding to RED tests for non-fatal stdin-mode turn errors.

[njfio]

Implemented non-fatal turn handling for interactive stdin mode.\n\nChanges:\n- now uses instead of propagating with .\n- Added helper that reports turn errors and returns .\n- Updated TTY path to use same helper for consistency.\n- Added regressions:\n - \n - \n\nVerification:\n- cargo test -p tau-coding-agent regression_resolve_interactive_turn_outcome_ -- --nocapture\n- cargo test -p tau-coding-agent regression_2548_run_interactive_with_runner_dispatches_stdin_mode -- --nocapture\n\nExpected outcome: provider/turn failures are surfaced, but interactive runtime stays alive for next prompt instead of exiting with status 1.

[njfio]

Implemented non-fatal turn handling for interactive stdin mode.

Changes:

• run_interactive_stdin now uses resolve_interactive_turn_outcome(dispatch_interactive_turn(...)) instead of propagating Err with ?.
• Added resolve_interactive_turn_outcome helper that reports turn errors and returns InteractiveLoopControl::Continue.
• Updated TTY path to use same helper for consistency.
• Added regressions:
  • regression_resolve_interactive_turn_outcome_converts_error_into_continue
  • regression_resolve_interactive_turn_outcome_preserves_exit_control

Verification:

• cargo test -p tau-coding-agent regression_resolve_interactive_turn_outcome_ -- --nocapture
• cargo test -p tau-coding-agent regression_2548_run_interactive_with_runner_dispatches_stdin_mode -- --nocapture

Expected outcome: provider/turn failures are surfaced, but interactive runtime stays alive for next prompt instead of exiting with status 1.