fix(agent): honor global JSON output format

[AlexMikhalev]

Summary

• Honors global --format json / --format json-compact for terraphim-agent guard, validate, and suggest.
• Covers both offline command handling and server-mode fallback paths.
• Preserves legacy subcommand --json compact one-line JSON output.
• Centralizes subcommand/global JSON mode compatibility in effective_json_output_mode(...).
• Adds regression tests for global format, pretty-vs-compact shape, server fallback errors, and legacy compact compatibility.

Test Plan

• cargo test -p terraphim_agent --test agent_format_contract_tests -- --nocapture — 10 passed
• cargo test -p terraphim_agent --test robot_search_output_regression_tests -- --nocapture — 5 passed
• cargo test -p terraphim_agent --test offline_mode_tests -- --nocapture — 24 passed
• cargo clippy -p terraphim_agent -- -D warnings — passed
• git diff --cached --check — clean before amend

Review

• Independent review deleg_5c4d6898 passed: no security concerns, no logic errors, no suggestions.

CI note

GitHub CI failures on this base are unrelated to the touched crates/terraphim_agent/* files: Firecracker VM creation fails, and workspace formatting failures are in pre-existing crates/terraphim_rlm/* files.

Refs terraphim/terraphim-ai#3172

[AlexMikhalev]

CI triage note:

The PR-specific focused verification passed locally:

• cargo test -p terraphim_agent --test agent_format_contract_tests — 8 passed
• cargo test -p terraphim_agent --test robot_search_output_regression_tests — 5 passed
• cargo test -p terraphim_agent --test offline_mode_tests — 24 passed
• cargo clippy -p terraphim_agent -- -D warnings — passed

Current failing GitHub checks appear unrelated to this PR's touched files:

• Firecracker VM lifecycle proof: VM create step fails with curl exit 22 against http://127.0.0.1:8080/api/vms.
• Rust build + test / lint-and-format: workspace cargo fmt --check reports pre-existing formatting diffs in crates/terraphim_rlm/src/rlm.rs, crates/terraphim_rlm/src/main.rs, and crates/terraphim_rlm/tests/*. This PR only changes crates/terraphim_agent/src/main.rs and adds crates/terraphim_agent/tests/agent_format_contract_tests.rs.

Base branch task/2668-terraphim-lsp-foundation latest runs are already failing on CI, so I am not folding unrelated RLM formatting/Firecracker infra fixes into this focused agent-format PR.

[AlexMikhalev]

Option A follow-up pushed in 2db074cbf.

Added:

• effective_json_output_mode(...) helper to remove duplicated JSON-mode compatibility logic.
• explicit --format json pretty-output contract test for guard.
• server-mode suggest JSON fallback error contract test.

Verification after the follow-up:

• cargo test -p terraphim_agent --test agent_format_contract_tests -- --nocapture — 10 passed
• cargo test -p terraphim_agent --test robot_search_output_regression_tests -- --nocapture — 5 passed
• cargo test -p terraphim_agent --test offline_mode_tests -- --nocapture — 24 passed
• cargo clippy -p terraphim_agent -- -D warnings — passed
• git diff --cached --check — clean

Independent review deleg_5c4d6898 passed with no security concerns, no logic errors, and no suggestions.

[AlexMikhalev]

Base CI unblocker update: PR #955 now has the ownable/code gates green on head 2791ccd16cede02e46bcb4cfb13b32b9d1fe291d:

• Native setup: success
• Native lint-and-format: success
• Firecracker Rust build + test: success
• Performance Benchmarks + Regression Check: success

Only remaining red check on #955 is Firecracker VM lifecycle proof, now diagnosed as fcctl create status: 401 and tracked separately as Gitea #3173 (bigbox/fcctl-web auth/infra).

Recommendation: merge/rebase the base CI unblocker (#955) before judging this PR's checks; this PR's older CI runs include stale base failures that #955 fixes.