feat: implement fast path resolution for Python environment managers

[eleanorjboyd]

• Added fast path resolution logic in SysPythonManager, VenvManager, CondaEnvManager, PipenvManager, and PyEnvManager to directly resolve persisted environments without waiting for full initialization.
• Enhanced error handling with logging for failed resolutions in the fast path.
• Introduced unit tests to validate the fast path behavior across different environment managers, ensuring correct resolution and background initialization handling.

[Copilot]

Pull request overview

This PR introduces a shared “fast path” for EnvironmentManager.get() so managers can immediately resolve a persisted environment selection (from workspace state) without waiting for full environment discovery to complete, while discovery continues in the background.

Changes:

• Added a reusable tryFastPathGet() helper (plus a shared getProjectFsPathForScope() accessor) and integrated it into built-in and tool-based managers.
• Updated get() implementations in SysPython/Venv/Conda/Pipenv/PyEnv managers to attempt fast resolution and trigger background initialization.
• Added unit tests for both the shared helper and manager-level behavior; updated startup-flow documentation to describe the new execution fork.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/managers/common/fastPath.ts	New shared helper implementing the fast-path resolution + background init orchestration.
src/managers/builtin/sysPythonManager.ts	Uses fast-path helper before falling back to standard initialization.
src/managers/builtin/venvManager.ts	Uses fast-path helper and starts background refresh via internalRefresh.
src/managers/conda/condaEnvManager.ts	Uses fast-path helper and runs discovery under withProgress.
src/managers/pipenv/pipenvManager.ts	Uses fast-path helper and runs discovery under withProgress.
src/managers/pyenv/pyenvManager.ts	Uses fast-path helper and runs discovery under withProgress.
src/test/managers/common/fastPath.unit.test.ts	New unit tests validating tryFastPathGet() behavior and background init semantics.
src/test/managers/fastPath.get.unit.test.ts	New unit tests validating fast-path behavior across multiple managers’ get() implementations.
docs/startup-flow.md	Documentation updates describing the new fast-path vs slow-path fork during startup/selection.

Comments suppressed due to low confidence (1)

src/managers/common/fastPath.ts:94

• Same as the traceError case above: traceWarn(... ${err}) will often lose the error stack/details. Prefer passing the error object as a separate argument to traceWarn so util.format can render it more helpfully.

            traceWarn(
                `[${opts.label}] Fast path resolve failed for '${persistedPath}', falling back to full init: ${err}`,
            );
        }
    }