Make repeated exact installs idempotent

[sdairs]

Summary

• treat an exact ClickHouse version that is already installed as a successful no-op
• emit the same already-installed message and --force hint used for partial version specs
• cover both exact and partial local-first installs with subprocess regression tests

Why

The local-first install path special-cased exact version specs by returning VersionAlreadyInstalled, while partial specs returned the matching installed version successfully. That made equivalent idempotent install requests produce different exit codes and telemetry outcomes.

Impact

Repeated exact-version installs now exit 0 without resolving or downloading anything, so scripts can safely use local install to ensure a version is present. The separate output-copy cleanup remains tracked by #328.

Stack

This PR is stacked on #348 and should be reviewed as the delta from codex/issue-329-batch-lsof.

Closes #339.

Validation

• cargo fmt --all --check
• cargo build -p clickhousectl --all-targets
• cargo test -p clickhousectl (571 tests)
• cargo clippy -p clickhousectl --all-targets -- -D warnings

[Copilot]

Pull request overview

This PR makes clickhousectl local install idempotent when the requested ClickHouse version is already installed, aligning behavior between exact (25.12.9.61) and partial (25.12) version specs in the local-first install path.

Changes:

• Treat an already-installed exact version spec as a successful no-op (exit 0) in install_local_first, matching partial-spec behavior.
• Reuse the existing “already installed” messaging (including the --force hint) for both exact and partial specs.
• Add subprocess regression coverage for both partial and exact installed-version no-op installs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/clickhousectl/src/version_manager/install.rs	Removes the exact-version “already installed” error in the local-first path so exact and partial specs both short-circuit successfully when a local match exists.
crates/clickhousectl/tests/local_install_local_first_test.rs	Adds a shared subprocess helper and a new regression test covering exact-version no-op behavior alongside the existing partial-version coverage.

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