fix: preserve conda environments on transient discovery failure

[StellaHuang95]

Context

Conda environment discovery runs the native finder (PET). When that refresh throws/rejects — a
transient failure — refreshCondaEnvs previously returned [], the same value it returns for a
genuine "no conda environments" result. CondaEnvManager could not tell the two apart, so a
transient failure replaced the known-good collection with [] and emitted spurious remove
events, making previously-discovered environments disappear from the UI until the next successful
refresh.

Root cause

refreshCondaEnvs collapsed two distinct outcomes into []:

• the native finder threw (discovery failed), and
• the native finder succeeded with zero environments.

Every manager discovery path (initialize, refresh, background get) then assigned
this.collection = result and emitted add/remove events, so a failure wiped the collection.

Fix

• refreshCondaEnvs now returns PythonEnvironment[] | undefined:
  • undefined = discovery failed (native finder threw/rejected).
  • an array (including []) = authoritative success; [] means genuinely no environments.
• The three manager discovery paths guard on undefined:
  • Failure: keep the existing collection and emit no removals, but still call loadEnvMap()
    so persisted global/workspace selections are restored — announcing only environments newly
    appended by that resolution (loadEnvMapPreservingCollection).
  • Success (including []): replace the collection and emit the normal add/remove events.

Legitimate deletion is unchanged: a successful [] is authoritative and removes stale environments
exactly as before. Only a thrown-failure undefined is treated as "preserve".

Tests

• condaUtils.refreshCondaEnvs.unit.test.ts: the utility returns undefined on a rejected refresh
  and [] on a successful empty discovery.
• condaEnvManager.resultPreservation.unit.test.ts:
  • a failed refresh/initialize preserves the known-good collection and emits no changes when
    nothing persisted resolves;
  • a successful [] empties the collection and emits removals; a successful non-empty result
    replaces the collection and emits removals + adds;
  • a failed discovery still restores a persisted global or workspace selection, emitting only its
    addition and never a removal, and retains it across get calls.

[StellaHuang95]

🔒 Automated review in progress — @StellaHuang95 is auto-reviewing this PR.

[StellaHuang95]

Warning · Non-blocking recommendation

📍 src/test/managers/conda/condaEnvManager.resultPreservation.unit.test.ts:255
The Set deduplicates addition names, so this test passes even if the background path emits duplicate add events. Assert the raw addition count and ordered names instead.

[verified]

[StellaHuang95]

Warning · Non-blocking recommendation

📍 src/managers/conda/condaEnvManager.ts:131
The same failure-result interpretation is repeated at all three discovery entry points, leaving the preservation invariant vulnerable to drift. Centralize applying a discovery result in a private helper, or otherwise keep the branches mechanically linked.

[verified]

[StellaHuang95]

Issue · Please address or respond

📍 src/managers/conda/condaEnvManager.ts:394
The Skeptic reproduced a race where successful background discovery finishes before the fast path, after which this silently inserts an environment into the already-announced authoritative collection. Reconcile this completion order by emitting an add when necessary or routing the late result through a common post-discovery application path, and add a gated regression test.

[verified]

[StellaHuang95]

Warning · Non-blocking recommendation

📍 src/managers/conda/condaEnvManager.ts:131
The prior maintainability concern is only partially addressed: recovery is shared, but three callers still independently interpret undefined. Centralize discovery-result interpretation so future call sites cannot accidentally collapse failure into an authoritative empty result while retaining their distinct success-event policies.

[verified]

[StellaHuang95]

Issue · Please address or respond

📍 src/managers/conda/condaEnvManager.ts:362
[verified] The prior successful-background/late-fast-path race remains: this assignment can publish the authoritative background collection before a later fast-path result is reconciled, leaving that result without an add event. Route the late result through a common reconciliation path and add a gated successful-background regression test.

[verified]

[StellaHuang95]

Warning · Non-blocking recommendation

📍 src/managers/conda/condaEnvManager.ts:127
The two prior centralization warnings are only partially addressed: initialization, refresh, and background initialization still interpret undefined independently. Centralize discovery-result interpretation while parameterizing each caller's distinct successful-result event policy.

[verified]

[StellaHuang95]

Warning · Non-blocking recommendation

The initialization, refresh, and background initialization paths still independently classify undefined. Centralize discovery-result classification while preserving each caller's distinct success-event policy. [verified]

[StellaHuang95]

Warning · Non-blocking recommendation

The pre-await known snapshot can become stale if another resolution appends and announces an environment while loadEnvMap() is awaiting, causing this helper to emit a duplicate add. Have loadEnvMap() report appended environments, or recheck announcement state before firing. [verified]