fix(markdown): avoid workspace-wide diagnostics refresh thread pileup

[sebthom]

Refresh diagnostics only for open Markdown buffers and debounce/serialize refresh requests and de-dupe in-flight refreshes. Make diagnostic pull non-blocking with timeout.

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This solves issue that threads like this are piling up:

"ForkJoinPool.commonPool-worker-262" #7921 [21548] daemon prio=5 os_prio=0 cpu=3781.25ms elapsed=124139.24s tid=0x00000226ec161850 nid=21548 waiting on condition  [0x0000002de7ffe000]
   java.lang.Thread.State: WAITING (parking)
        at jdk.internal.misc.Unsafe.park(java.base@21.0.9/Native Method)
        - parking to wait for  <0x0000000765001c88> (a java.util.concurrent.CompletableFuture$Signaller)
        at java.util.concurrent.locks.LockSupport.park(java.base@21.0.9/LockSupport.java:221)
        at java.util.concurrent.CompletableFuture$Signaller.block(java.base@21.0.9/CompletableFuture.java:1864)
        at java.util.concurrent.ForkJoinPool.compensatedBlock(java.base@21.0.9/ForkJoinPool.java:3740)
        at java.util.concurrent.ForkJoinPool.managedBlock(java.base@21.0.9/ForkJoinPool.java:3723)
        at java.util.concurrent.CompletableFuture.waitingGet(java.base@21.0.9/CompletableFuture.java:1898)
        at java.util.concurrent.CompletableFuture.get(java.base@21.0.9/CompletableFuture.java:2072)
        at org.eclipse.wildwebdeveloper.markdown.MarkdownDiagnosticsManager.refreshFile(MarkdownDiagnosticsManager.java:264)
        at org.eclipse.wildwebdeveloper.markdown.MarkdownDiagnosticsManager.lambda$0(MarkdownDiagnosticsManager.java:229)
        at org.eclipse.wildwebdeveloper.markdown.MarkdownDiagnosticsManager$$Lambda/0x00000007c1ddeac0.visit(Unknown Source)
        at org.eclipse.core.internal.resources.Resource.lambda$1(Resource.java:156)
        at org.eclipse.core.internal.resources.Resource$$Lambda/0x00000007c0dccd40.visit(Unknown Source)
        at org.eclipse.core.internal.resources.Resource.lambda$0(Resource.java:124)
        at org.eclipse.core.internal.resources.Resource$$Lambda/0x00000007c07c32e8.visitElement(Unknown Source)
        at org.eclipse.core.internal.watson.ElementTreeIterator.doIteration(ElementTreeIterator.java:85)
        at org.eclipse.core.internal.watson.ElementTreeIterator.doIteration(ElementTreeIterator.java:90)
        at org.eclipse.core.internal.watson.ElementTreeIterator.doIteration(ElementTreeIterator.java:90)
        at org.eclipse.core.internal.watson.ElementTreeIterator.doIteration(ElementTreeIterator.java:90)
        at org.eclipse.core.internal.watson.ElementTreeIterator.doIteration(ElementTreeIterator.java:90)
        at org.eclipse.core.internal.watson.ElementTreeIterator.doIteration(ElementTreeIterator.java:90)
        at org.eclipse.core.internal.watson.ElementTreeIterator.doIteration(ElementTreeIterator.java:90)
        at org.eclipse.core.internal.watson.ElementTreeIterator.iterate(ElementTreeIterator.java:129)
        at org.eclipse.core.internal.resources.Resource.accept(Resource.java:133)
        at org.eclipse.core.internal.resources.Resource.accept(Resource.java:92)
        at org.eclipse.core.internal.resources.Resource.accept(Resource.java:156)
        at org.eclipse.core.internal.resources.Resource.accept(Resource.java:144)
        at org.eclipse.wildwebdeveloper.markdown.MarkdownDiagnosticsManager.refreshAllOpenMarkdownFiles(MarkdownDiagnosticsManager.java:225)
        at org.eclipse.wildwebdeveloper.markdown.MarkdownLanguageClient.lambda$1(MarkdownLanguageClient.java:186)
        at org.eclipse.wildwebdeveloper.markdown.MarkdownLanguageClient$$Lambda/0x00000007c1dde3d8.run(Unknown Source)
        at java.util.concurrent.CompletableFuture$AsyncRun.run(java.base@21.0.9/CompletableFuture.java:1804)
        at java.util.concurrent.CompletableFuture$AsyncRun.exec(java.base@21.0.9/CompletableFuture.java:1796)
        at java.util.concurrent.ForkJoinTask.doExec(java.base@21.0.9/ForkJoinTask.java:387)
        at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(java.base@21.0.9/ForkJoinPool.java:1312)
        at java.util.concurrent.ForkJoinPool.scan(java.base@21.0.9/ForkJoinPool.java:1843)
        at java.util.concurrent.ForkJoinPool.runWorker(java.base@21.0.9/ForkJoinPool.java:1808)
        at java.util.concurrent.ForkJoinWorkerThread.run(java.base@21.0.9/ForkJoinWorkerThread.java:188)

   Locked ownable synchronizers:
        - None