fix(setup): implement isSetup() to check for matched.json marker

[karthik120710]

Description:

Summary

Fixes the unimplemented isSetup() guard in tool/internal/setup/setup.go.

Resolves #

Changes

• Implemented isSetup() to check for the presence of matched.json (written by store() at the end of a successful Setup run) instead of always
  returning false
• Added unit tests for both the "not set up" and "already set up" cases

Why

Without this fix, the guard at Setup() line 149 would never short-circuit, causing setup to re-run on every build once the guard is wired in —
resulting in redundant re-initialization.

Test Plan

• go test ./tool/internal/setup/... -run TestIsSetup passes
• Full setup package tests pass: go test ./tool/internal/setup/...

Notes from the guidelines:

• No changelog entry needed — this is a bug fix for an internal/unshipped guard with no user-facing behavior change. You can prefix the title with
  [chore] if reviewers flag it, or ask a maintainer to add the Skip Changelog label.
• The title follows the [component] description format required by the guidelines.
  isSetup() is unimplemented and hard-codes false #512

[github-actions]

The title of this pull request does not match the conventional commits format.
Please update the title, as apprioriate.

Refer to the CONTRIBUTING.md file for more information.

[github-actions]

The title of this pull request does not match the conventional commits format.
Please update the title, as apprioriate.

Refer to the CONTRIBUTING.md file for more information.

[karthik120710]

Hi @txabman42 @kakkoyun can u review my pr

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.88%. Comparing base (e88e6f5) to head (dbc0275).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##           main     #513       +/-   ##
=========================================
+ Coverage      0   62.88%   +62.88%     
=========================================
  Files         0       62       +62     
  Lines         0     4766     +4766     
=========================================
+ Hits          0     2997     +2997     
- Misses        0     1525     +1525     
- Partials      0      244      +244     

Flag	Coverage Δ
tool	62.88% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[txabman42]

Checking the file existence is not enough. The setup file might be related to a previous configuration, making not possible to do the setup again.

This problem is being discussed in this issue