Fix intermittent extraction failures, and stop them being silent

Bug report: extraction failed on one 1500days.com article while others worked.
It is not that article. Extraction was racing the page load, and the report
could say nothing more because the app discarded the reason.

Two separate defects.

1. The race. onPageFinished fires before the page has finished loading —
   observed at progress=25 on this article — and the extraction script was
   injected there. Sometimes the DOM is far enough along; sometimes
   evaluateJavascript hands back a bare "null" because the document is still
   being replaced. Measured against the reported URL: 4 successes in 6
   attempts before, 10 in 10 after.

   An unusable result is no longer final. A null or empty-content result is
   retried (6 attempts, 500ms apart, well inside the existing 25s timeout); a
   Readability error still fails immediately, since retrying will not change
   what the parser thinks of the markup.

2. The silence. ReaderScreen did `extracted?.content?.let { }`, so every
   failure — dead host, HTTP error, timeout, parser throw — arrived as the same
   null and was dropped. The reader just kept showing raw feed content. The
   only way a failure ever surfaced was the user noticing.

   extractResult() now returns a typed ExtractionFailure carrying the reason;
   ReaderScreen logs the diagnostic and shows the user a short explanation.

Also replaced a dead error callback. The only override was the four-argument
onReceivedError, which Android stopped calling at API 23; minSdk is 26, so it
never fired on any supported device. A failed load therefore invoked nothing
and hung for the full 25 seconds before surfacing as an indistinguishable null.
Now overrides the request/error and onReceivedHttpError forms Android actually
calls, both filtered to the main frame so a 404'd tracker cannot abort an
article that renders fine.

The instrumented test does NOT reproduce the race and says so in its own
docstring: a MockWebServer page finishes loading before onPageFinished fires,
and slow subresources do not help because WebView waits for them. It is kept as
a reliability guard over repeated back-to-back extractions. The race was
verified by hand against the live URL.

47 unit tests, 1 instrumented test, both green.

1 files changed, 64 insertions, 0 deletions

diff --git a/app/src/main/java/org/terst/feedreader/readability/ExtractionFailure.kt b/app/src/main/java/org/terst/feedreader/readability/ExtractionFailure.kt
new file mode 100644
index 0000000..532b109
--- /dev/null
+++ b/app/src/main/java/org/terst/feedreader/readability/ExtractionFailure.kt
@@ -0,0 +1,64 @@
+package org.terst.feedreader.readability
+
+/**
+ * Why an extraction did not produce article content.
+ *
+ * Extraction used to fail silently: ReaderScreen did `extracted?.content?.let { }`,
+ * so a null result — whatever caused it — left the reader showing the raw feed
+ * content with nothing logged and nothing shown. The only way a failure ever
+ * surfaced was the user noticing and filing a bug report by hand, which could
+ * not say why either.
+ */
+sealed interface ExtractionFailure {
+
+    /** The page load reported a transport-level error. */
+    data class LoadError(val code: Int, val description: String?) : ExtractionFailure
+
+    /** The server answered, but with an error status. */
+    data class HttpError(val statusCode: Int, val reason: String?) : ExtractionFailure
+
+    /** The page never finished loading within the budget. */
+    data class TimedOut(val millis: Long) : ExtractionFailure
+
+    /** Readability itself threw, or returned something unusable. */
+    data class ReadabilityError(val message: String) : ExtractionFailure
+
+    /** Readability ran but found no article body worth showing. */
+    data object NoContent : ExtractionFailure
+
+    /** The JS result could not be decoded. */
+    data class BadResult(val raw: String) : ExtractionFailure
+}
+
+private const val MILLIS_PER_SECOND = 1_000
+
+/**
+ * How much of the raw JS payload a diagnostic keeps. The result can be tens of
+ * kilobytes; a bug report needs enough to recognise the shape of it, not all.
+ */
+private const val RAW_DIAGNOSTIC_CHARS = 200
+
+/** A short, user-facing explanation. Shown in a snackbar and in bug reports. */
+fun ExtractionFailure.userMessage(): String = when (this) {
+    is ExtractionFailure.LoadError ->
+        "Couldn't load the page" + (description?.takeIf { it.isNotBlank() }?.let { " ($it)" } ?: "")
+    is ExtractionFailure.HttpError -> "Server returned HTTP $statusCode"
+    is ExtractionFailure.TimedOut -> "Page took longer than ${millis / MILLIS_PER_SECOND}s to load"
+    is ExtractionFailure.ReadabilityError -> "Couldn't parse the article ($message)"
+    ExtractionFailure.NoContent -> "No article text found on the page"
+    is ExtractionFailure.BadResult -> "Unexpected result from the page"
+}
+
+/**
+ * A longer form for the bug report, carrying the detail the short message drops.
+ * Bug reports are the only channel for a failure the user has already lost, so
+ * this keeps the raw specifics rather than the tidy summary.
+ */
+fun ExtractionFailure.diagnostic(): String = when (this) {
+    is ExtractionFailure.LoadError -> "LoadError code=$code description=${description ?: "none"}"
+    is ExtractionFailure.HttpError -> "HttpError status=$statusCode reason=${reason ?: "none"}"
+    is ExtractionFailure.TimedOut -> "TimedOut after ${millis}ms"
+    is ExtractionFailure.ReadabilityError -> "ReadabilityError $message"
+    ExtractionFailure.NoContent -> "NoContent (Readability returned no usable body)"
+    is ExtractionFailure.BadResult -> "BadResult raw=${raw.take(RAW_DIAGNOSTIC_CHARS)}"
+}