fix: align BroadcastPartition routing in LinkConfig with broadcast semantics

[Ma77Ball]

What changes were proposed in this PR?

LinkConfig.toPartitioning's BroadcastPartition() arm was building channels with fromWorkerIds.zip(toWorkerIds), producing a 1:1 pairing rather than the full sender-by-receiver cross product that broadcast semantics imply.
The sibling ChannelConfig.generateChannelConfigs in the same package already emits a cross product for the broadcast arm, so the two helpers disagreed on routing for the same inputs. As with #4799 for OneToOnePartition,
List.zip also silently truncated the surplus side on unequal-length inputs. This PR replaces the zip with the same flatMap / map cross product already used by the HashPartition, RangePartition, and
UnknownPartition arms in this file, which fixes the semantics and removes the silent truncation hazard.

Any related issues, documentation, or discussions?

Closes: #4802

How was this PR tested?

The two LinkConfigSpec cases that pinned the previous behavior (with comments explicitly inviting this realignment) were flipped to assert the cross product: 3 senders by 3 receivers now produces 9 channels, and the
Asymmetric 3 by 2 case now produces 6 channels rather than silently truncating to 2. sbt scalafmtAll was run; CI should pass the existing LinkConfigSpec, ChannelConfigSpec, and downstream scheduling specs.

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude Opus 4.7 in compliance with ASF

[Ma77Ball]

/request-review @aglinxinyuan

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.85%. Comparing base (1497806) to head (c53431f).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5032      +/-   ##
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+ Coverage     42.77%   42.85%   +0.08%     
- Complexity     2193     2199       +6     
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  Files          1045     1031      -14     
  Lines         39985    38156    -1829     
  Branches       4217     4007     -210     
============================================
- Hits          17102    16352     -750     
+ Misses        21824    20787    -1037     
+ Partials       1059     1017      -42     

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[Copilot]

Pull request overview

This PR fixes LinkConfig.toPartitioning so that BroadcastPartition() produces the full sender×receiver channel cross product (broadcast semantics) instead of incorrectly using zip (1:1 pairing with silent truncation on length mismatch). This aligns LinkConfig routing behavior with the existing ChannelConfig.generateChannelConfigs behavior in the same package and resolves issue #4802.

Changes:

• Update LinkConfig.toPartitioning’s BroadcastPartition() arm to generate channels via cross product (flatMap/map) rather than zip.
• Update LinkConfigSpec broadcast test cases to assert the cross-product routing for symmetric and asymmetric worker list sizes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
amber/src/main/scala/org/apache/texera/amber/engine/architecture/scheduling/config/LinkConfig.scala	Fix broadcast routing to emit sender×receiver channel cross product instead of zip pairing.
amber/src/test/scala/org/apache/texera/amber/engine/architecture/scheduling/config/LinkConfigSpec.scala	Update broadcast partition specs to match corrected broadcast semantics (including unequal list sizes).

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[aglinxinyuan]

LGTM!