[BUG] ExtensionLoader pinned to first ClassLoader; plugin-contributed SPI extensions invisible

- Severity: Medium
- Location:
`shenyu-spi/src/main/java/org/apache/shenyu/spi/ExtensionLoader.java:102-107` (`putIfAbsent` keeps first loader), `:78` (classloader stored at construction), `:268-271` (`loadDirectory` calls `classLoader.getResources` only on pinned loader)
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Description:
`getExtensionLoader(clazz, cl)` caches `ExtensionLoader` by `Class` only. `putIfAbsent` keeps the first loader (typically app classloader). `loadDirectory` calls `classLoader.getResources("META-INF/shenyu/")` on that pinned loader. A later call with a plugin `ClassLoader` returns the cached app-classloader loader, so the plugin's `META-INF/shenyu/...` SPI files are never enumerated.
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Impact:
Plugin-contributed SPI extensions silently don't load; `getJoin(name)` throws `IllegalArgumentException` at runtime for the contributed name. The SPI subsystem cannot be extended by hot-deployed plugins.
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Suggested fix:
Enumerate resources across all registered classloaders (merge results, last-wins or error-on-dup), or key `LOADERS` by `(clazz, classLoader)`.
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Confidence: Medium
- Related existing: none

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._

Start in shenyu-spi/src/main/java/org/apache/shenyu/spi/ExtensionLoader.java, focusing on getExtensionLoader, the cached classloader, loadDirectory, and getJoin. Reproduce loading a plugin-contributed META-INF/shenyu/ resource with a later ClassLoader. Done means those resources are enumerated and the contributed extension name no longer causes getJoin(name) to throw.

Written by the indexing model from the issue text.