fix(middleware): reset lastIndex before testing a command pattern

[alter]

Closes #1058

matchesPattern in src/middleware/builtin.ts calls pattern.test(candidate) directly on the
RegExp a caller passed to app.command(). RegExp#test() advances lastIndex on a pattern that
carries the g or y flag, and that state lives on the RegExp object itself, not in the call.
Since the same pattern object is reused for every incoming command (the middleware closure
captures it once), a command registered with a g or y flagged pattern matches on the first
invocation, fails to match on the second, matches again on the third, and so on, alternating
forever. The affected listener silently never runs on the failing invocations, so ack() never
gets called and Slack shows the interaction as timed out.

Fix resets pattern.lastIndex to 0 right before calling test(), so every invocation starts from
the same state regardless of what a previous invocation left behind. This only matters for
patterns with the g or y flag; resetting lastIndex on a pattern without either flag is a no-op
since test() ignores it. The two other places in builtin.ts that match a pattern
(matchConstraints and matchMessage) use String#match() instead of RegExp#test(), which does
not carry this same statefulness, so they were not affected and were not touched.

Tested:

1. New test in test/unit/middleware/builtin.spec.ts: registers matchCommandName(/hi/g) once,
   invokes the resulting middleware twice with the same matching command, and asserts next()
   gets called both times. Confirmed it fails against unmodified code (second invocation
   never calls next()) and passes after the fix.
2. npm test (build, lint, type tests, unit test coverage): 500 passing, 0 failing, same as
   before this change plus the 1 new test.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2e5657b

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[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @alter to sign the Salesforce Inc. Contributor License Agreement.

[alter]

recheck

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.38%. Comparing base (672c9fb) to head (2e5657b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3090   +/-   ##
=======================================
  Coverage   94.38%   94.38%           
=======================================
  Files          43       43           
  Lines        7426     7431    +5     
  Branches      704      704           
=======================================
+ Hits         7009     7014    +5     
  Misses        408      408           
  Partials        9        9           

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[AmyScript]

Thanks for the fix! The test is a proper regression test, two invocations with fresh args, assserting next is calledOnce on each which failed before the change and passes after. LGTM!