fix(clap_complete): handle hyphenated bin names in bash completions

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What does this PR try to solve?

Closes #6386. For a binary named my-app, the Bash generator normalized the root function name to my__app but also converted the hyphen in the root command path into a __subcmd__ separator. As a result, the command-state strings could not match the generated nested-subcommand handlers, so completion stopped after entering the command. Normalize the root path consistently with the function name.

Notes to reviewers

The existing custom_bin_name Bash snapshot covers a hyphenated binary and nested subcommands; the other affected Bash snapshots now assert the corrected handler names.

Validation: cargo test -p clap_complete --test testsuite (77 passed); cargo fmt --check.

LLM involvement: AI-assisted implementation and tests; the regression is covered by the generated-script snapshots.

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Our LLM policy specifically says that anything touching completion shell scripts cannot be AI generated.