runtime: only deduct assist credit for arenas during GC

[wingrez]

CL 617876 has changed the original behavior. This modification will restore it.

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Shengwei Zhao:

Patch Set 1:

(2 comments)

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Message from Shengwei Zhao:

Patch Set 2:

(1 comment)

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Message from Michael Knyszek:

Patch Set 3: Code-Review+2

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Message from Michael Knyszek:

Patch Set 3: Auto-Submit+1 Commit-Queue+1

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Message from Go LUCI:

Patch Set 3:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-07-22T16:13:37Z","revision":"0d42e067b5e7a30b38347faf18ef52d9e5437c52"}

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Message from Michael Knyszek:

Patch Set 3:

(2 comments)

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Message from Michael Knyszek:

Patch Set 3:

(1 comment)

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Message from Michael Knyszek:

Patch Set 3: -Commit-Queue

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Patch Set 3:

This CL has passed the run

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Patch Set 3: LUCI-TryBot-Result+1

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Message from Shengwei Zhao:

Patch Set 4:

(3 comments)

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Message from Mark Freeman:

Patch Set 4: Code-Review+1

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