Flake: HedgedObjectReadSourceTest.HedgeOpenFailureReleasesSlot

https://storage.googleapis.com/cloud-cpp-community-publiclogs/logs/google-cloud-cpp/16412/acd756cac86341327010db887393348eaabb9831/ubuntu-22.04-cmake-oldest-deps-__default__/log-412127b2-3ff0-4b33-a2dc-363f228a0a1b.txt

```
Step #2 - "build.sh": [ RUN ] HedgedObjectReadSourceTest.HedgeOpenFailureReleasesSlot
Step #2 - "build.sh":
Step #2 - "build.sh": GMOCK WARNING:
Step #2 - "build.sh": Uninteresting mock function call - returning default value.
Step #2 - "build.sh": Function call: Close()
Step #2 - "build.sh": Returns: 104-byte object
Step #2 - "build.sh": NOTE: You can safely ignore the above warning unless this call should not happen. Do not suppress it by blindly adding an EXPECT_CALL() if you don't mean to enforce the call. See https://github.com/google/googletest/blob/main/docs/gmock_cook_book.md#knowing-when-to-expect-useoncall for details.
Step #2 - "build.sh": /workspace/google/cloud/storage/internal/hedged_object_read_source_test.cc:316: Failure
Step #2 - "build.sh": Value of: result
Step #2 - "build.sh": Expected: code is equal to OK and message is anything
Step #2 - "build.sh": Actual: 304-byte object <90-78 6E-CB 76-55 00-00 00-00 00-00 00-00 00-00 A8-7E 8B-CA 76-55 00-00 28-C1 DA-32 FF-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 ... 30-10 00-7C 21-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00>, whose status is UNAVAILABLE: open failed, with a code that isn't equal to OK, but a message that is anything
Step #2 - "build.sh":
Step #2 - "build.sh": [ FAILED ] HedgedObjectReadSourceTest.HedgeOpenFailureReleasesSlot (30 ms)

```

Start with internal/hedged_object_read_source_test.cc around line 316 and run HedgedObjectReadSourceTest.HedgeOpenFailureReleasesSlot using the linked build log for comparison. Trace why the test receives UNAVAILABLE instead of OK and identify the cause of the intermittent failure. Done means the test no longer flakes and its expected result is reliable.

Written by the indexing model from the issue text.