[type:refactor] externalize Hystrix to plugin-store - #7499

Summary

This PR externalizes the Hystrix plugin out of the main repository: the runtime plugin (shenyu-plugin-hystrix) and its Spring Boot starter (shenyu-spring-boot-starter-plugin-hystrix) are removed, and every built-in reference to Hystrix in the default distribution is cleaned up. A migration doc (docs/plugin-store-hystrix.md) is added explaining how existing deployments opt back in via the external plugin-store starter.

The externalization is executed consistently and the repository still compiles cleanly. Verified:

1. No dangling references to the removed code. A code search across the default branch returns 0 references to the removed artifact shenyu-spring-boot-starter-plugin-hystrix, and 0 references to the removed classes HystrixPlugin, HystrixCommand, HystrixBuilder, HystrixPluginDataHandler. The two reactor pom.xml files (shenyu-plugin-fault-tolerance/pom.xml and shenyu-spring-boot-starter-plugin/pom.xml) correctly drop the hystrix modules from <modules>, and the bootstrap / integrated-test-http / e2e shenyu-cm.yml dependencies are removed in lockstep.
2. Compatibility contract is deliberately preserved, as stated in the new doc. At the PR head shenyu-common still ships HystrixHandle, HystrixIsolationModeEnum (and their tests), and PluginEnum.HYSTRIX (HYSTRIX(130, 0, "hystrix")). These let the external plugin-store and pre-existing admin rule data keep working during the cutover window — a reasonable, well-documented decision.
3. Admin surface updated consistently. EnumServiceImpl.list() now filters PluginEnum.HYSTRIX out of pluginEnums and drops the hystrixIsolationModeEnums block; EnumServiceTest is updated to match (removed hystrixIsolationModeEnums from both the non-null-key and support-key streams).
4. Seed data removed uniformly across all dialects. plugin (id 9), resource (1529402639284355099 and the 9 button rows ...5435654..5662), permission (...306626620 and ...76629888..896), and namespace_plugin_rel (...808282186 → plugin_id 9) rows are deleted from mysql, ob, og, oracle, pg, and the h2 admin schema, all consistently.
5. LICENSE and fallback config cleaned. The three hystrix-* entries are removed from the dist LICENSE, and /fallback/hystrix is removed from application.yml and both shenyu-cm.yml files. A code search confirms no Java fallback mapping still references hystrix, so those path removals leave no dangling dispatch.

On CI

The check list shows many fail entries, but the sampled build (shenyu-integrated-test-http) job log ends with ##[error]The operation was canceled. — a cancellation, not a compile or test failure. The root build, Analyze (java), check-license-header, changes, and native-sql (pg/oracle/ob) jobs are green, and the failures span unrelated modules (apache-dubbo, grpc, combination, websocket, e2e-rabbitmq/rocketmq/grpc/springcloud, …). This is the standard infrastructure/cancellation pattern rather than a regression introduced by this PR. Please re-run before merging, but the red is not a reason to block.

Non-blocking suggestions

• RELEASE-NOTES.md: Dropping a formerly built-in plugin from the default bootstrap distribution is a user-visible behavior change. The new docs/plugin-store-hystrix.md covers the cutover, but a short ### Behavior Changes entry under ## Unreleased in RELEASE-NOTES.md is the canonical signal downstream users watch for. Recommended to add.
• HttpTestControllerTest.java import swap (org.apache.commons.lang.RandomStringUtils → org.apache.commons.lang3.RandomStringUtils): this looks unrelated to the Hystrix change at first glance, but it is almost certainly a necessary fix — removing the hystrix starter drops hystrix-core 1.5.18's transitive commons-lang (lang2) from this module's classpath, so the lang2 RandomStringUtils would no longer resolve. If commons-lang (lang2) is still available transitively via shenyu-common, the swap is harmless noise; otherwise it is the right fix. Either way it is fine; just confirming the intent is captured.
• Historical website plugin docs: the new doc correctly describes the externalized plugin, but the existing ShenYu website plugin documentation still presents Hystrix as a built-in plugin. That is naturally the plugin-store repo's responsibility to migrate, so it is out of scope here — just flagging it as a follow-up so the two repos stay consistent after this lands.

Decision

Clean, consistent externalization with the compatibility contract intact and no dangling references. No blocking issues. Approved.