chore: sync Java client with Apify OpenAPI spec v2-2026-09-28T115051Z

[Pijukatel]

Syncs the client's pinned OpenAPI specification version from v2-2026-09-10T091137Z to the latest published v2-2026-09-28T115051Z (apify-docs PRs #3002, #3013, #2975).

The spec delta over this range is entirely documentation:

• Actor-run charge idempotency-key header is now required: true and documents a 3-minute key expiration (#3013).
• Task publish-limit prose reworded: up to 10 published tasks per Actor, 100 per account (was "fewer than 50 per Actor") (#3002).
• API rate-limit table in the spec's top-level description reshuffled (new 35/100/350 req/s tiers) (#2975).

No functional code change is required: RunClient.charge() already always sends the idempotency-key header, auto-generating one when the caller omits it, so the header's new required: true is already satisfied. Neither the old publish limit nor the rate-limit figures were encoded anywhere in this client.

Changes

• Version.java: bump API_SPEC_VERSION to v2-2026-09-28T115051Z and CLIENT_VERSION to 0.6.5 (patch).
• pom.xml: <version> synced to 0.6.5.
• README.md: synced the dependency snippets and the Versioning section's example values.
• CHANGELOG.md: new 0.6.5 entry (kept to a short summary per review feedback).

Verification

mvn spotless:check, mvn compile spotbugs:check, and the release javadoc build (mvn -Prelease package) all clean. Full offline unit suite (118 tests) and the live integration suite (including the Test examples doc-snippet checks) pass against APIFY_TOKEN.