Avoid retrying refused connections in DownloadManager

[jjj-n]

Description

• Do not treat ConnectException as retryable in DownloadManager.
• Preserve manager-level retries for other socket failures, unknown hosts, and content-length mismatches.
• Add a regression test verifying that a suppressed ConnectException invokes the downloader only once, even when manager retries are configured.

Motivation

HttpConnector already retries connection attempts with exponential backoff. When its final ConnectException reached DownloadManager, the default five manager-level retries replayed that entire sequence, causing downloader rewrites to offline endpoints to take much longer to fail.

In an end-to-end reproduction using an offline rewritten endpoint, the failure time decreased from 83.329 seconds with Bazel 9.2.0 to 13.921 seconds with this change.

Fixes #30746.

Testing

bazel test //src/test/java/com/google/devtools/build/lib/bazel/repository/downloader:DownloaderTestSuite

The complete suite passed: 1/1 test target, with the test process completing in 138.3 seconds.

Build API Changes

No

Checklist

• I have added tests for the new use cases (if any).
• I have updated the documentation (if applicable).

Release Notes

RELNOTES: Downloader rewrites to endpoints that refuse connections no longer repeat the connector's full retry sequence.

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[jjj-n]

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