fix: add guard for disposed JCEF browser in sendToWebview

[RomneyDa]

Summary

• Adds an isClosed guard before calling executeJavaScript in sendToWebview to prevent NPE when the JCEF browser is already disposed
• Broadens the catch clause from IllegalStateException to Exception to handle NPE and other errors that propagate from deeper in JCEF

Fixes #8085, #9159

Test plan

• Open a Continue sidebar panel in IntelliJ, close the project, and verify no NPE is logged
• Verify normal webview messaging still works when the browser is active

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Summary by cubic

Prevent NPEs when the JCEF browser is disposed by guarding sendToWebview, broadening error handling, and logging a warning when messages are dropped. This avoids errors on project close and keeps webview messaging stable and debuggable.

• Bug Fixes
  • Return early if browser.cefBrowser.isClosed and log the message type before skipping executeJavaScript.
  • Catch Exception instead of IllegalStateException to handle NPEs and other JCEF errors.

^{Written for commit 01d876b. Summary will update on new commits.}

[continue]

Docs Review

No documentation updates needed for this PR.

Reason: This change is an internal bug fix for JCEF browser disposal handling in the JetBrains extension. It adds a defensive guard and broader exception handling to prevent NPEs on project close—purely implementation-level details that don't affect user-facing behavior, configuration, or usage patterns.

[cubic-dev-ai]

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