fix(DesktopContentExplorer): PSDesktopExplorerWindow this-escape (#2444)

[natechadwick-intsof]

Summary

Eliminates javac this-escape on PSDesktopExplorerWindow (residual of #2384 under parent #2045 / #2200).

Changes

• PSDesktopExplorerStateProvider is now a static nested class (does not capture outer this).
• PSJavaBridge is lazy-init via getBridge() (double-checked locking) so constructors/field initializers do not publish this.
• setJavaBridge() installs the bridge through getBridge().
• Non-serializable collaborators (bridge, myStateProvider) marked transient.
• Unit tests for static provider, post-ctor null bridge, stable lazy bridge, and transient fields.

No product behavior change — bridge is still created before install on the page window.

Fixes #2444
Parent trackers: #2045, #2200
Operator: Grok: night-issue-prs (model grok-4.5)

Test plan

• cd modules/DesktopContentExplorer && ../../mvnw.cmd clean install — BUILD SUCCESS
• Tests run: 86, Failures: 0, Errors: 0, Skipped: 0 (includes 6 new PSDesktopExplorerWindowTest)
• No new compiler warnings attributable to this change
• Erlang-style self-review: no path I/O; change-class companions (unit tests); lazy init preserves install-time bridge identity

Gates evidence

Gate	Result
Module standalone clean install	BUILD SUCCESS
Unit tests	86 / 0 fail
this-escape mitigation	static nested + lazy bridge
Human QA	Not required (pure tech-debt, no UI/install behavior change)

Co-Authored by Grok Build using grok-4.5 with agent main.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• modules/DesktopContentExplorer/src/main/java/com/percussion/cx/javafx/PSDesktopExplorerWindow.java
• modules/DesktopContentExplorer/src/test/java/com/percussion/cx/javafx/PSDesktopExplorerWindowTest.java

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_{Review guidance: REVIEW.md from base branch main}