gopls/internal/golang: support variadic functions and constructors in generated tests

[IlyasYOY]

Add IsVariadic flag to function metadata and expand variadic arguments with ... in test templates. Update constructor handling to propagate variadic information. Refactor import collection helper and add varargs test cases.

Fixes golang/go#76682

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Hongxiang Jiang:

Patch Set 1: Commit-Queue+1

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Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-12-06T00:30:06Z","revision":"bfabb7b86c59f8f2b6f0a4191dc6910eb3c22861"}

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Message from Hongxiang Jiang:

Patch Set 1: -Commit-Queue

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Patch Set 1: LUCI-TryBot-Result+1

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Message from Hongxiang Jiang:

Patch Set 1:

(2 comments)

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