Hint available build for unavailable exact installs

[sdairs]

Closes #341

Summary

• preserve downloads for exact versions that have historical release artifacts
• hint the newest indexed build in the same major.minor series when an exact rolling build is unavailable
• preserve the generic no-match error when the series has no available build

Tests

• cargo fmt --all --check
• cargo test -p clickhousectl
• cargo clippy -p clickhousectl --all-targets -- -D warnings

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[Copilot]

Pull request overview

Improves the local version resolver’s error path for exact-version installs by preserving existing historical downloads when present, and by providing a more actionable hint (nearest available build in the same minor series) when an exact rolling build can’t be resolved from the index.

Changes:

• Enhance resolve_exact() to, on exact-version miss, look up the newest available version in the same major.minor series and return a dedicated error that includes a retry hint.
• Introduce Error::ExactVersionUnavailable with a multi-line, actionable message.
• Add unit tests covering the new error construction and ensuring the message includes the expected retry guidance.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
crates/clickhousectl/src/version_manager/resolve.rs	Adds exact-version miss handling that searches the major.minor series for the newest available build and returns a more actionable error.
crates/clickhousectl/src/error.rs	Introduces a dedicated error variant and test to surface a “nearest available” hint for unavailable exact installs.

Suppressed comments (1)

crates/clickhousectl/src/error.rs:168

• Update this assertion to match the adjusted ExactVersionUnavailable wording so the test continues to validate the exact rendered message.

        assert_eq!(
            error.to_string(),
            "build 26.2.8.7 is no longer available for download.\n\
             Nearest available in the 26.2 series: 26.2.20.4 \
             (try `clickhousectl local install 26.2`)"
        );

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[sdairs]

Review triage note: Copilot’s suppressed summary suggestion to update the ExactVersionUnavailable assertion is already satisfied on the current head; the test asserts the exact rendered message and passes. No additional change is needed for that point.