refactor(tui): unify API format representation across all apps

[heliar-k]

Summary

Add a Codex API format picker to the TUI provider form, unify API format representation across all apps under a single ApiFormat enum, and fix a bug where provider meta would revert to old values after editing.

Key Changes

1. Codex API Format Picker

• Add API format picker to Codex provider form (non-official providers)
• Picker shows 2 options: OpenAI Responses (default) and OpenAI Chat (local routing)
• Reuses the existing picker overlay infrastructure from Claude/Gemini

2. Enum Unification

• Rename ClaudeApiFormat → ApiFormat (describes generic wire protocols, not Claude-specific)
• Replace separate claude_api_format + codex_api_format fields with a single api_format field
• Unify field identifier: ProviderAddField::ClaudeApiFormat + ::CodexApiFormat → ::ApiFormat

3. Naming Consistency

• Proxy: get_claude_api_format → get_provider_api_format
• CLI constants: CLAUDE_API_FORMAT_* → API_FORMAT_* (shared by both paths)
• i18n: tui_label_claude_api_format → tui_label_api_format
• CLI function pairs retain symmetric naming: claude_* ↔ codex_*

4. Codex Sponsor Templates

• Codex sponsor templates now emit apiFormat meta, aligning CLI output with TUI serializer

5. Code Cleanup

• Remove dead ApiFormat enum from proxy/types.rs
• Remove unreachable Codex branch from render arm

6. Bug Fix: Provider Meta Revert After Edit

• Fix bug where changing a provider's apiFormat (e.g., from openai_chat to Anthropic) in the TUI would appear to save, but revert to the old value after restart
• Root cause: update_provider_meta() removed the entire meta key when the meta object became empty, producing JSON without meta. On deserialization this yielded meta: None, which triggered the merge logic in ProviderService::update() to preserve the old meta (including the old apiFormat)
• Fix: keep "meta": {} in the JSON output even when empty, so deserialization produces Some(ProviderMeta::default()) and the merge logic correctly treats the form output as authoritative
• Added end-to-end regression test covering the full TUI form → ProviderService::update → AppState reload cycle

Impact

• 28 files changed, 462 insertions, 282 deletions
• Eliminates dual-field synchronization complexity
• Makes API format semantics consistent across Claude/Codex/Gemini/OpenCode/Hermes/OpenClaw
• Fixes provider meta revert bug that affected any edit that emptied the meta object

Tests

• All 29 api_format tests pass (including new test_exact_tui_form_api_format_roundtrip regression test)
• All 8 codex_local_routing tests pass
• All 24 provider_add_form_codex tests pass
• cli_provider_templates_match_tui_serializer_output (previously failing, now fixed)

Verification

cd src-tauri
cargo build
cargo fmt --check
cargo clippy
cargo test --lib -- api_format
cargo test --lib -- codex_local_routing
cargo test --lib -- provider_add_form_codex
cargo test --test provider_commands