Fix log message to correctly reference invalid venv environment not conda (#788)

## Problem

The `findVirtualEnvironments` function in
`src/managers/builtin/venvUtils.ts` was logging misleading warning
messages. When encountering an invalid virtual environment (missing
prefix, executable, or version), it would log:

```
Invalid conda environment: {...}
```

However, this function is specifically designed to find and process
**venv** environments, not conda environments. The function correctly
filters for `NativePythonEnvironmentKind.venv` on line 179, making the
conda reference in the warning message incorrect and confusing for
developers debugging environment discovery issues.

## Solution

Updated the warning message on line 183 to accurately reflect the
environment type being processed:

```typescript
// Before
log.warn(`Invalid conda environment: ${JSON.stringify(e)}`);

// After
log.warn(`Invalid venv environment: ${JSON.stringify(e)}`);
```

## Impact

This change provides accurate debugging information when invalid venv
environments are encountered, making it easier for developers and users
to understand which type of environment manager is reporting the issue.
The fix is minimal and surgical, changing only the log message text
without affecting any functionality.

All existing tests continue to pass, confirming that this change doesn't
introduce any regressions.

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Author: Copilot

src/managers/builtin/venvUtils.ts

@@ -180,7 +180,7 @@ export async function findVirtualEnvironments(
 
     for (const e of envs) {
         if (!(e.prefix && e.executable && e.version)) {
-            log.warn(`Invalid conda environment: ${JSON.stringify(e)}`);
+            log.warn(`Invalid venv environment: ${JSON.stringify(e)}`);
             continue;
         }