feat(custom-endpoint): support Anthropic Messages protocol for custom endpoints

[JanCong]

Summary

• Add API protocol selection (OpenAI / Anthropic) to the hermes model custom endpoint setup wizard, with auto-detection from URL patterns (e.g. /anthropic suffix)
• Persist api_mode in custom_providers config entries so the protocol choice survives across sessions
• Read api_mode from custom_providers in resolve_custom_provider() instead of hardcoding openai_chat transport
• Generalize /v1 auto-stripping for Anthropic protocol to all providers (previously only applied to OpenCode) — the Anthropic SDK appends /v1/messages automatically, so a base_url ending in /v1 would produce .../v1/v1/messages (HTTP 404)

Motivation

The custom endpoint setup wizard only offered "OpenAI-compatible" endpoints. Users who needed the Anthropic Messages protocol (e.g. MiniMax /anthropic, LiteLLM proxies, custom gateways) had to manually edit config.yaml to add api_mode: anthropic_messages. This PR makes that a first-class option in the interactive setup flow.

Closes #12137, closes #6209

Test plan

• hermes model → Custom endpoint → enter an Anthropic-protocol URL (e.g. https://api.minimaxi.com/anthropic) → verify auto-detection prompts "Anthropic Messages"
• hermes model → Custom endpoint → enter a plain URL → verify manual protocol selection appears
• Select Anthropic Messages with a /v1-suffixed URL → verify /v1 is auto-stripped with a note printed
• Check ~/.hermes/config.yaml → custom_providers entry has api_mode: anthropic_messages
• Check model.api_mode is correctly set in config
• /model in-chat switch to a custom Anthropic endpoint → verify no /v1/v1/messages 404
• Existing OpenAI-compatible custom endpoints continue to work unchanged

🤖 Generated with Claude Code

[teknium1]

Closing as superseded by #25068.

Triage notes (high confidence):
Main hermes_cli/main.py:3659 already calls _prompt_custom_api_mode_selection for interactive api_mode picking in _model_flow_custom — landed via merged #25068 (feat(custom): prompt and persist explicit api_mode).

Thanks for the contribution — the underlying problem this PR addresses has been resolved by the linked PR on current main. If you believe this was closed in error, please comment and we'll reopen.

(Bulk-closed during a CLI PR triage sweep.)