fix(auth): remove unneeded OAuth flows

[patrasap0908]

Related issue

• Preemptive toolset auth triggers OAuth redirect on every agent invocation #5327

Problem

During agent invocation, OAuth redirection happens for every toolset (that returns an AuthConfig) regardless of whether the user message requires tool invocation or not.

Solution

This PR defers the OAuth flow to tool invocation time, which already works as expected.

Testing plan

Unit tests

• I have added or updated unit tests for my change.
• All unit tests pass locally.

End-to-end test results:

============================================================= 5457 passed, 1 skipped, 2231 warnings in 292.50s (0:04:52) ==============================================================

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Since _resolve_toolset_auth() will simply defer auth flows to tool invocation time, there is no need to handle the topic of framework- and tool-level credential file path mismatch in this PR. Once consensus is reached, a separate PR can be opened for it later.