fix: preserve colon arguments in bash completions

[puneetdixit200]

Fixes #2376.

Summary

• rejoin Bash words split around : before Cobra calls __complete
• keep ValidArgsFunction receiving URI-like values such as local:/ as the active argument
• add a Bash-driven regression test for the split local : / case

Tests

• GOMODCACHE=$PWD/.gomodcache GOCACHE=$PWD/.gocache go test ./ -run TestBashCompletionV2RejoinsColonWordbreak -count=1 (failed before the fix)
• GOMODCACHE=$PWD/.gomodcache GOCACHE=$PWD/.gocache go test ./ -run 'TestBashCompletionV2(RejoinsColonWordbreak|WithActiveHelp)' -count=1
• GOMODCACHE=/tmp/cobra-2376-gomodcache GOCACHE=/tmp/cobra-2376-gocache make all
• PATH=/tmp/cobra-2376-bin:$PATH GOMODCACHE=/tmp/cobra-2376-gomodcache GOCACHE=/tmp/cobra-2376-gocache golangci-lint run --new-from-rev=upstream/main --verbose
• git diff --check

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