refactor(insurance): extract EligibilityResult into its own class

[danigrim]

Summary

Moves EligibilityResult out of AppointmentService into a top-level class in the same package so the insurance eligibility response type no longer hangs off the scheduling service. Routine cleanup with no behavior change.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/b5c88d06ed92443da61216d5d5cf8137
Open in Devin Desktop: https://app.devin.ai/desktop/session/b5c88d06ed92443da61216d5d5cf8137?variant=devin
Requested by: @danigrim

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[devin-ai-integration]

CI first responder: build check fixed

Failing check: build — https://github.com/COG-GTM/demos-coghealth-ehr-api/actions/runs/33874537487 (commit 104df3f)

Root cause: this PR extracted EligibilityResult out of AppointmentService into its own top-level class, but InsuranceGateway.verifyEligibility still built the result via AppointmentService.EligibilityResult.builder() at line 41, so javac failed with cannot find symbol (only the method's return type on line 29 had been updated).

Change: one line — AppointmentService.EligibilityResult.builder() → EligibilityResult.builder() (same package, no import needed). No behavior, audit-logging, or PHI-handling changes. Commit a249455.

Verification: compiled all of src/main/java on Java 21 with the project's resolved Maven classpath and Lombok annotation processing — clean before-and-after check: fails with exactly InsuranceGateway.java:41: error: cannot find symbol without the fix, compiles with 0 errors with it. The repo has no src/test, so mvn test has no tests to run. (Note: mvn compile itself could not run end-to-end on my sandbox — Maven Central rate-limited this IP with HTTP 429 while resolving maven-resources-plugin deps — so I verified via javac against the same dependency set; GitHub Actions will run the real Maven build on this commit.)

A human reviewer still owns approval and merge.

Written by Devin