apollo_consensus: replace numbers with consts in StreamHandler tests

[guy-starkware]

Note

Low Risk
Test-only refactor with no production logic changes; risk is limited to potential minor test expectation mistakes.

Overview
Refactors stream_handler_test.rs to replace repeated 0 literals used for round/stream/message IDs with named constants (ROUND_0, STREAM_ID_0, MESSAGE_ID_0) to improve readability and reduce magic numbers in tests.

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[reviewable-StarkWare]

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[guy-starkware]

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This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• apollo_dashboard: add panels for StreamHandler metrics #13034
• apollo_consensus: replace numbers with consts in StreamHandler tests #13031 👈 (View in Graphite)
• apollo_consensus: add StreamHandler alerts for peer eviction and buffer full #12937
• apollo_consensus: add metrics in StreamHandler for evicted peers and full buffer #12935
• apollo_consensus: limit number of buffered messages in each stream #12744
• apollo_consensus: add max_message_buffer_size to StreamHandler config #12739
• apollo_consensus: replace key with peer_id and stream_id in internal functions #12737
• apollo_consensus: StreamHandler keeps streams for each peer on a separate LRU cache #12736
• main

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[asmaastarkware]

@asmaastarkware reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on guy-starkware).