Rust: Isolate hostless runtime cache

[ellismg]

Overview

Separate the non-bundled hostless runtime cache from the bundled CLI cache. Non-bundled in-process builds now extract to <platform cache>/github-copilot-sdk/runtime/<version>, while bundled CLI extraction remains at cli/<version>.

This prevents stale hostless-runtime cleanup from deleting a same-version bundled copilot executable used by another application. COPILOT_CLI_EXTRACT_DIR retains its existing direct-directory semantics.

Changes

• Share hostless runtime path computation between build.rs and runtime resolution.
• Document the distinct cache paths on macOS, Linux, and Windows.
• Add regression coverage proving runtime cleanup cannot delete or overwrite the bundled CLI directory.
• Update focused non-bundled resolution tests for the new namespace.

Validation

• rustup run nightly-2026-04-14 cargo fmt --check
• cargo test --no-default-features --lib cache_paths::tests
• Focused cli_resolution_test cases for conventional extraction, runtime resolution, and COPILOT_CLI_EXTRACT_DIR
• cargo clippy --no-default-features --lib -- -D warnings
• cargo clippy --all-features --all-targets -- -D warnings

[Copilot]

Copilot review overview

🟢 Approval recommended

The shared path logic consistently isolates runtime cleanup while preserving custom extraction-directory behavior.

Review tier: Balanced
Findings: None

What changed in this PR

Separates Rust hostless runtime artifacts from bundled CLI cache entries to prevent destructive cleanup collisions.

Changes:

• Introduces shared runtime cache-path computation.
• Moves non-bundled artifacts from cli/<version> to runtime/<version>.
• Adds regression coverage and documents platform paths.

File	Description
rust/​src/​cache_paths.rs	Centralizes cache-path logic and isolation tests.
rust/​build/​in_process.rs	Uses the runtime-specific cache path.
rust/​src/​resolve.rs	Resolves extracted runtimes through shared logic.
rust/​src/​lib.rs	Registers the cache-path module.
rust/​tests/​cli_resolution_test.rs	Updates expected extraction location.
rust/​README.md	Documents distinct cache namespaces.

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[ellismg]

This will be helpful for us on the copilot app team. Right now we can run into an issue without this fix which causes the CLI we use as part of our dev builds when runing out of main to get deleted and replaced with an incompatible version, breaking things until we restart the dev version of the app and redownload the correct version.