add fourslash test for declare missing method quickfix (#37782)

[oppong07]

This PR adds a fourslash test that verifies the quick fix for declaring a missing private method on a class.

Summary

tests: add tests/cases/fourslash/codeFixAddMissingMethod.ts to exercise the missing-method quick fix.
No runtime code changes — the implementation already exists in src/services/codefixes/fixAddMissingMember.ts.
How to run locally

npx hereby runtests --tests=codeFixAddMissingMethod

Checklist

There is an associated issue in the Backlog milestone
Code is up-to-date with main
I've run tests locally: npx hereby runtests --tests=codeFixAddMissingMethod
New unit test added
Fixes #37782

[oppong07]

@microsoft-github-policy-service agree

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

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