fix: shield: Make start and stop idempotent

[matthiasgeihs]

Explanation

Currently calling ShieldController.start repeatedly will create several event subscriptions.
This PR proposes to make start and stop idempotent, so that calling start won't have an effect if the controller is already started, and the same for stop, respectively.

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

---

Note

Make ShieldController.start/stop idempotent; update tests and changelog.

• Controller:
  • Make start()/stop() idempotent in src/ShieldController.ts by adding #started flag and guarding subscribe/unsubscribe.
• Tests:
  • Update ShieldController.test.ts to use setupCoverageResultReceived and add a test ensuring repeated start()/stop() calls are tolerated.
• Changelog:
  • Add Unreleased note: "Make start and stop idempotent" in CHANGELOG.md.

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[lwin-kyaw]

LGTM!