[Bug] Provider-specific HTTP 400 rate-limit responses stop processing

Description

In some organizations, LLM limits are configured for a one-minute window rather than a long-term quota. When the provider returns HTTP 400 with a JSON detail containing rate limit exceeded, OpenCode currently treats the response as a terminal processing failure instead of retrying the request.

This stops the current session even though the request may succeed after the one-minute limit window resets.

Steps to reproduce

1. Configure an organization-managed LLM provider with a one-minute request or token limit.
2. Send a prompt until the provider limit is reached.
3. Receive an HTTP 400 response with a JSON body containing a detail field whose value is rate limit exceeded.
4. Observe that OpenCode stops processing instead of entering the existing retry flow.

Expected behavior

OpenCode should retry this provider-specific HTTP 400 rate-limit response using the existing retry flow, while keeping generic HTTP 400 client errors non-retryable.

The behavior should be opt-in per provider because HTTP 400 rate-limit responses are non-standard:

provider:
  my-provider:
    options:
      retry400RateLimit: true

When the option is disabled or absent, the current non-retryable behavior should remain unchanged.

Environment

• OpenCode: current dev branch; implementation is in PR #40239
• Provider: organization-managed LLM provider with a one-minute limit window
• Scope: provider/API response handling; not OS-specific