feat(tui): add config key support to provider create/update forms

[letv1nnn]

Summary

Add config key (config map) support to the TUI's Create Provider and Update Provider forms.

Previously, both forms only handled credentials and hardcoded config: HashMap::default(), making it impossible to set provider config keys like ANTHROPIC_BASE_URL or OPENAI_BASE_URL via openshell term. The CLI's --config KEY=VALUE flag worked correctly, only the TUI surface was affected.

Related Issue

#1798

Changes

• Create Provider form: Added config key/value input section with Tab-based navigation between key and
  value fields, auto-commit on Tab, and Ctrl+D to delete entries
• Update Provider form: Added config key/value editing with existing config hydrated from the provider
  on form open, preserving config that the user doesn't modify
• Spawn functions: spawn_create_provider and spawn_update_provider now plumb the form's config map
  into the gRPC request instead of sending empty maps

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

Screenshots

You can run cargo run -p openshell-cli -- term to try it out yourself.

Provider Details now lists config keys:

Updating Provider Details

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[letv1nnn]

I have read the DCO document and I hereby sign the DCO.

[letv1nnn]

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