fix/chat: Wait for 'ready' before sending theme data.

[dominiccooney]

We used to inject themes using CSSOM injected by the client. We moved this code to the Agent to share it with VS, etc. and used DOMContentLoaded as the signal to do it. This races with webview React, which may be responding to this signal by DOMContentLoaded, but is not guaranteed to be.

Now we watch for the webview to send the 'ready' event before dispatching the initial theme update message.

Fixes CODY-3796, BUGS-126, BUGS-233, BUGS-154.

Test plan

Manually tested:

• Start and restart the IDE a bunch of times, the theme should be applied.
• Switch the theme, that should still work.

[dominiccooney]

• fix/chat: Wait for 'ready' before sending theme data. #2371 👈
• main

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[dominiccooney]

We have a ./gradlew check failure, this fails for me locally with and without this commit. Maybe main is flaky or broken? The commit history tells the tale.

Here's the failure I'm getting locally:

AllSuites > com.sourcegraph.cody.edit.DocumentCodeTest.testGetsWorkingGroupLens FAILED
    com.intellij.testFramework.TestLoggerFactory$TestLoggerAssertionError at TestLoggerFactory.java:403
        Caused by: java.lang.Throwable at Logger.java:370

AllSuites > com.sourcegraph.cody.edit.DocumentCodeTest.testUndo FAILED
    java.lang.NoClassDefFoundError at DaemonListeners.java:343
        Caused by: java.lang.NoClassDefFoundError at DaemonListeners.java:343
            Caused by: java.lang.ExceptionInInitializerError at TestLoggerFactory.java:403

AllSuites > com.sourcegraph.cody.edit.DocumentCodeTest.testShowsAcceptLens FAILED
    java.lang.NoClassDefFoundError at DaemonListeners.java:343
        Caused by: java.lang.NoClassDefFoundError at DaemonListeners.java:343
            Caused by: java.lang.ExceptionInInitializerError at TestLoggerFactory.java:403

AllSuites > com.sourcegraph.cody.edit.DocumentCodeTest.testAccept FAILED
    java.lang.NoClassDefFoundError at DaemonListeners.java:343
        Caused by: java.lang.NoClassDefFoundError at DaemonListeners.java:343
            Caused by: java.lang.ExceptionInInitializerError at TestLoggerFactory.java:403

4 tests completed, 4 failed

[pkukielka]

LGTM