chore: update Go server SDK to 7.15.5

[devin-ai-integration]

Summary

Bumps github.com/launchdarkly/go-server-sdk/v7 from an exact pin of v7.15.4 to the latest within-major release v7.15.5.

SDK v7.15.5 declares go 1.25.0, so go mod tidy raises this module's directive from go 1.24.0 (+ toolchain go1.24.1) to go 1.25.0. Consequently the README's "requires Go 1.24 or higher" and the CI setup-go version are updated to 1.25.

Verified by running the example against a real SDK key: client initializes on SDK 7.15.5, streams, and evaluates the flag. No deprecated SDK APIs in use; go build and go vet are clean. CI build-and-run passes.

Link to Devin session: https://app.devin.ai/sessions/17c48091dc31418fbb97b133dd299665
Requested by: @jsonbailey

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Note

Low Risk
Dependency and toolchain bump only; no auth, data, or application logic changes.

Overview
Updates github.com/launchdarkly/go-server-sdk/v7 from v7.15.4 to v7.15.5 with no changes to application source.

Because the newer SDK targets Go 1.25, go.mod moves from go 1.24.0 (and drops the toolchain go1.24.1 pin) to go 1.25.0. The README minimum Go version and CI setup-go go-version are aligned to 1.25 so local builds and the workflow use the same toolchain.

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[devin-ai-integration]

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[devin-ai-integration]

@cursor review