fix(provider): classify transport send failures for fallback

[Alix-007]

Summary

• classify common transport send/request failures as retriable timeout-style fallback errors
• cover connection reset, connection refused, no route to host, unexpected EOF, and TLS handshake timeout
• add focused classifier regression tests for these transport patterns

Closes #1419.

Testing

• go test ./pkg/providers -run TestClassifyError_TransportSendPatterns -count=1

[Alix-007]

Closing this to avoid keeping another duplicate branch in the queue.

After re-checking issue #1419 and the current PR queue, this branch is not the best line to keep active:

• there are already overlapping open PRs on the same transport-classification topic, especially fix(providers): classify transport errors as retriable for fallback chain #1465 and fix(providers): classify transport layer errors as retriable for fallback chain #1430
• this PR only addresses the classifier half of Fallback chain aborts on retriable OpenRouter transport resets #1419, while the issue description also calls out candidate-provider resolution behavior
• with multiple near-identical open PRs, keeping this one open adds queue noise instead of helping review throughput

If the maintainers later prefer this exact patch shape, it can be reopened, but for now I think reducing duplicate PRs is the better move.