fix: prevent duplicate dotenv key mass-deletion from keys view

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Summary

• Fixes a critical data-loss bug in dotenv key management UI where removing one duplicate key deleted all lines with that key.
• Adds an index-aware removal helper so the UI removes exactly the selected row.
• Adds regression tests covering duplicate-key removal behavior.

Bug and impact

In the dotenv Keys view, clicking Remove on a key used key-based deletion (removeDotenvKey) that removes every matching key line. For files with duplicate keys (common during merges or staged rotations), one click could silently delete multiple values and persist them on save.

Root cause

The UI rendered duplicate entries distinctly by row index, but removal was implemented by key name only, collapsing all duplicates into one deletion operation.

Fix

• Added removeDotenvEntryAt(content, entryIndex) in src/lib/dotenv-parse.ts.
• Updated file-workspace.tsx to pass the displayed row index to removal and call removeDotenvEntryAt.
• Kept legacy removeDotenvKey unchanged for existing callers.

Validation

• Added src/lib/dotenv-parse.test.ts with regression coverage for duplicate keys and preservation of comments/blank lines.
• Ran: npm test -- src/lib/dotenv-parse.test.ts (passes).