fix(app): tolerate partial provider failures in get_all_provider_models()

[tivris]

Summary

get_all_provider_models() uses .collect::<Result<Vec<_>>>() which fails the entire operation if any single configured provider returns an error. This means a stale credential for one provider (e.g. Bedrock) blocks model fetching for all other providers, causing confusing errors after an unrelated provider login like:

● GoogleAIStudio configured successfully
● ERROR: Bearer token is required in API key field

The function's docstring already describes the correct behavior (return partial results, only error when all providers fail), but the implementation doesn't match.

This PR replaces the fail-fast collection with a loop that:

• Collects successful provider results
• Logs tracing::warn! for individual provider failures
• Only returns an error when every configured provider fails

Follows the same pattern used in the MCP service connection handler (forge_services/src/mcp/service.rs).

Fixes #2935

Test plan

• RUSTFLAGS='-Dwarnings' cargo check -p forge_app compiles clean
• cargo test -p forge_app passes
• Configure two providers, invalidate one's credentials, verify the other still works for model selection

[tusharmath]

This implementation was recently changed. The problem was when the API key was invalid and more than one provider was activated the user would not be able to view the models of the invalid provider.

One option that I propose is - we could return a Vec of Result of models and on the UI, first show the error and then print the model selector with the models.

:model
[**:**:**] Error: Something bad happened
MODEL  PROVIDER  CONTEXT WINDOW
...    ...       ...    
...    ...       ...    
...    ...       ...    

[tivris]

Updated the implementation per your suggestion. get_all_provider_models() now returns Vec<Result<ProviderModels>> so individual provider errors reach the UI. Changes:

• Per-provider errors displayed to stderr (keeps --porcelain stdout clean)
• Full error chain shown via alternate Display format
• If every provider fails, callers still get a real error (not a silent empty list)
• 4 call sites in ui.rs updated accordingly