CAMEL-23912: Address review feedback on ThemeTest fix

[gnodet]

Claude Code on behalf of gnodet

Summary

Follow-up to #24475 — addresses review feedback from Copilot and @ammachado that landed after the original PR was merged.

Changes:

• Handle null classloader (Copilot): Theme.class.getClassLoader() can legally return null for bootstrap-loaded classes. Now null-checked before calling getResourceAsStream(), falling through to the TCCL fallback instead of throwing an NPE.
• Soften javadoc wording (@ammachado suggestion Handle Jetty's removal of method ServletContextHandler.addFilter(String,String,int) #1): Changed from "guaranteed to have access" to accurately describe the fallback behavior, so a future reader doesn't strip the TCCL fallback as dead code.
• Revert ConcurrentHashMap to HashMap (@ammachado suggestion Camel ssh component CAMEL-4713 #2): All CACHE access is already inside synchronized methods, so the concurrent map added no real safety — just a misleading signal.

Test plan

• All 6 ThemeTest tests pass.
• mvn formatter:format impsort:sort produces no diff.

🤖 Generated with Claude Code

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🧪 CI tested the following changed modules:

• dsl/camel-jbang/camel-jbang-plugin-tui

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🔬 Scalpel shadow comparison — Scalpel: 2 tested, 1 compile-only — current: 2 all tested

Maveniverse Scalpel detected 3 affected modules (current approach: 2).

⚠️ Modules only in Scalpel (1)

• camel-launcher

Skip-tests mode would test 2 modules (1 direct + 1 downstream), skip tests for 1 (generated code, meta-modules)

Modules Scalpel would test (2)

• camel-jbang-plugin-tui
• camel-launcher-container

Modules with tests skipped (1)

• camel-launcher

ℹ️ Shadow mode — Scalpel observes but does not affect test execution. Learn more

All tested modules (3 modules)

• Camel :: JBang :: Plugin :: TUI
• Camel :: Launcher
• Camel :: Launcher :: Container

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