fix(oauth): forget a deleted user's token in the latest installation

[WilliamBergamin]

Summary

This pull request makes FileInstallationStore.deleteInstallation({ userId }) forget that user's token in the latest installation.

• Before, deleting a user removed only the user-<id>-* files. When that user was also the latest installer, app-latest kept their token, so fetchInstallation({ userId }) still returned it.
• Now the user in app-latest is reduced to { id } when it matches; the bot and other users are untouched.
• Historical app-<timestamp> copies are unchanged, since fetchInstallation never reads them.

Testing

1. Install an app with user scopes through the OAuth flow, so you are the latest installer.
2. Call installationStore.deleteInstallation({ teamId, userId }) with your user ID.
3. Confirm fetchInstallation({ teamId, userId }) returns no user token (before this change it still did).

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 2423818

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@slack/oauth	Patch

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[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.46%. Comparing base (1fabb45) to head (2423818).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2784      +/-   ##
==========================================
+ Coverage   89.40%   89.46%   +0.05%     
==========================================
  Files          65       65              
  Lines       10545    10548       +3     
  Branches      478      478              
==========================================
+ Hits         9428     9437       +9     
+ Misses       1086     1080       -6     
  Partials       31       31              

Flag	Coverage Δ
cli-hooks	89.31% <91.66%> (-0.04%)	⬇️
cli-test	89.31% <91.66%> (-0.04%)	⬇️
logger	89.31% <91.66%> (-0.04%)	⬇️
oauth	89.31% <91.66%> (-0.04%)	⬇️
socket-mode	89.31% <91.66%> (-0.04%)	⬇️
web-api	89.31% <91.66%> (-0.04%)	⬇️
webhook	89.31% <91.66%> (-0.04%)	⬇️

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