Fix zero key length panic in Prism

[shunping]

This PR mainly fixes the "zero key length" panic occurred in our tests.

• First, it fixes the preprocessing of TestStream to preserve KV coder. This is necessary for the next step to retrieve key coder.
• Second, it applies the same approach as Fix flaky Prism Loopback test #36927 to obtain key coder and add keyBytes when constructing pending elements in TestStream

In addition, it shows a concerning situation that a panic in watermark eval loop does not trigger a pipeline failure. To correct that, we change the order of two defer calls, so that the panic is handled before a premature channel closing.

Four tests are expected to fail and we will sickbay them for further investigation(tracking in #36984).

• ParDoTest$BundleFinalizationTests.testBundleFinalizationWithState
• ParDoTest$StateTests.testMapStateNoReadOnComputeIfAbsentAndPutIfAbsentInsertsElement
• ParDoTest$TimerTests.testOutputTimestamp
• ParDoTest$TimerTests.testOutputTimestampWithProcessingTime

[github-actions]

Assigning reviewers:

R: @lostluck for label go.

Note: If you would like to opt out of this review, comment assign to next reviewer.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).