ci: exclude JDTLS workspaces from language-server cache

[TyceHerrman]

Summary

• keep caching downloaded language-server installations
• exclude JDTLS project workspaces from the shared GitHub Actions cache
• force each CI job to initialize its own Eclipse/Buildship runtime state

Why

Serena currently caches all of ~/.serena/language_servers/static, including
EclipseJDTLS/workspaces. Those directories contain mutable, project-specific
Eclipse and Buildship state. Restoring that state into another job can make Java
test behavior depend on an earlier job rather than only on the checked-out
revision and current runner.

This is an independent cache-correctness change. It does not claim that restored
workspace state caused the Windows JDTLS hang; the diagnostics work in #1786 is
still needed to classify that stall.

The cache action supports multiple glob paths and leading ! exclusions. Its
cache version also incorporates the configured paths, so the new definition
does not match the old broad cache archive.

#1214 intentionally preserves JDTLS workspaces across local Serena restarts. #1576 established stale JDTLS workspace state as a real correctness risk. This change preserves that user behavior while preventing GitHub Actions from restoring project-derived workspace state across revisions that share the same runner checkout path.

Validation

• workflow YAML parses successfully
• git diff --check
• actionlint reports only diagnostics already present on main

[opcode81]

Is this ready?

[TyceHerrman]

Not yet I need to review it further still

[TyceHerrman]

@opcode81 this is ready for review now afaik

[opcode81]

@TyceHerrman sorry, I had accidentally closed the wrong PR.

[opcode81]

Thanks for your contribution.