feat(apex-testing): new command SFDX: Edit Apex Test Suite that includes add and remove functionality - W-23257457

[peternhale]

What does this PR do?

Adds Apex Test Suite editing support — the ability to add and remove tests from a suite
directly from VS Code — and delivers it as a single unified "Edit Apex Test Suite"
command (sf.apex.test.suite.edit).

Phase 1 — Remove Tests from Apex Test Suite:

• Implemented apexTestSuiteRemove: queries current TestSuiteMembership records, presents
  a multi-select quick pick of current members, removes selected classes via Tooling API
• Tooling API deletes TestSuiteMembership records individually (batch endpoint unsupported)
• Used Effect.tryPromise for all tooling calls so rejections surface as typed errors
• Graceful handling when no suites exist or a suite is empty (info message instead of
  silent cancellation)
• Added E2E coverage, docs, and i18n (English + Japanese)

Phase 2 — Unify add/remove into a single Edit command:

• Replaced sf.apex.test.suite.add and sf.apex.test.suite.remove with
  sf.apex.test.suite.edit
• New UX: multi-select picker showing all test classes with current suite members
  pre-checked; toggling adds or removes in one step
• gatherEditOptions() queries suite membership and all ApexClasses in parallel, uses
  ApexClass ID matching to correctly pre-check existing members
• applyEdits() handles add/remove as parallel Tooling API operations
• Updated command registration, i18n, E2E specs, README, and walkthrough docs

Also includes:

• Fixed pre-existing lint warnings in salesforcedx-apex (Function → proper signatures,
  import order in logService.ts)
• Added fileSystemHandler.ts to knip ignore list (false positive)

What issues does this PR fix or reference?

@W-23257457@

Functionality Before

No way to remove tests from a suite in VS Code. Adding tests required a separate "Add Tests
to Apex Test Suite" command with no visibility into current membership.

Functionality After

Single "SFDX: Edit Apex Test Suite" command. Multi-select picker shows all test classes with
current suite members pre-checked — check/uncheck to add or remove in one operation.

Testing

• No Apex test suites available -> "No Apex test suites found." info notification ✅
• Create Apex test suite ✅
• Be able to check boxes to add tests to a test suite ✅
• Be able to uncheck boxes to remove tests from a test suite ✅
• Remove all tests such that you end up with an empty test suite ✅
• Do both adding and removing Apex tests in one run of the edit suite command ✅
• Click "OK" with no changes to the selection ✅
• Try to run empty Apex test suite (now possible after removing all tests from the suite) -> get error notification saying there's nothing to run ✅
• Cancellation when selecting suite by clicking in the Editor View ✅
• Cancellation when selecting tests by clicking in the Editor View ✅