goTest CodeLens: preserve buildFlags as array when debugging tests

[vrv3814]

debugTestAtCursor CodeLens previously joined go.buildFlags into a single space-separated string before passing them to the debug adapter. This corrupted flag values that contain spaces, such as -ldflags "-X pkg.version=v1.0": after joining, where delve sees -X and pkg.version=v1.0 as separate arguments and fails with with multiple packages, -o must refer to a directory or /dev/null.
Meanwhile, the run-test CodeLens passes each flag as a separate argument to go test, so the same go.buildFlags worked for "run test" but broke "debug test".

Fix

Changes are made to pass buildFlags as an array to startDebugging. The existing maybeJoinFlags helper in goDebugConfiguration.ts already handles both shapes: it joins for delve < 1.22.2 and preserves the array for newer delve, so each element reaches delve as a separate token. Also I've omitted buildFlags from the debug config when empty to keep behavior parity with the previous "" sentinel.

Fixes #3933

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