fix(flags): load shared definitions without a secret key

[dustinbyrne]

💡 Motivation and Context

Like PostHog/posthog-python#1039, a worker consuming shared feature flag definitions should not need a privileged secret key. Existing initialization gates prevent that supported cache-consumer configuration from evaluating flags locally.

Changes

• Permit the configured definition cache provider to supply definitions without a secret/personal API key.
• Keyless consumers read the shared cache directly, without invoking the fetch-decision callback or publishing definitions. They cannot claim fetch leadership they are unable to use.
• With privileged credentials, retain the existing decision flow: false reads the cache; true fetches definitions and publishes them. Direct definition requests remain authenticated.
• Add public-entry regressions, update active credential/provider docs, and include scoped patch release metadata.

Cache-only readers bypass the shared loader specification's decision-first provider step; credentialed consumers retain that step.

Only posthog-server changes. Provider-only setups still require .localEvaluation(true); the provider setter retains the existing explicit opt-in. Android/client-core defaults and behavior are unchanged.

💚 How did you test it?

• ./gradlew :posthog-server:test spotlessCheck :posthog-server:lint :posthog-server:apiCheck: passed.
• Server suite: 586 tests, zero failures/errors/skips; focused public provider/loader suites: 96 tests passed.
• Public tests cover startup, local evaluation, reload/polling, explicit opt-out, provider shutdown and snapshot/matching-version preservation.
• Shared-leadership and reader regressions fail against the prior loader and pass with the fix.
• Server-only local validation on JDK 17; Android/Compose suites were not run locally.

Release scope

Patch release for posthog-server. Existing signatures, capture defaults and wire formats are unchanged.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with Pi worker agents and reviewed by separate fresh-context Pi reviewers. A simplify pass covered the scoped source/tests/docs before final local validation. Human review remains required.