fix(semanticdb-javac): report analysis exceptions as WARNING instead of ERROR

[rpalcolea]

reportException() was calling reporter.error() which uses Diagnostic.Kind.ERROR, causing javac to exit non-zero and fail the build. The surrounding catch block explicitly states 'we don't want to stop the compilation' — this change makes the implementation match the intent.

Adds SemanticdbReporter.warning() for plugin-internal failures, and updates SemanticdbTaskListener.reportException() to use it. Intentional error paths (e.g. -no-relative-path:error mode) are unchanged.

Fixes builds for projects with partial classpaths (e.g. Apache Spark) where CompletionFailure for anonymous inner classes like DataType was breaking compileJava entirely.

Fixes #861

Test plan

What we're testing

SemanticdbTaskListener.reportException() previously emitted Diagnostic.Kind.ERROR,
causing javac to exit non-zero and fail the build when semanticdb-javac hit an internal
exception. After the fix it emits Kind.WARNING, so the build succeeds with partial
semanticdb output.

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New unit test

PartialClasspathSuite.scala in tests/unit/src/test/scala/tests/.

How it triggers reportException()

The test pre-creates the semanticdb output path (META-INF/semanticdb/example) as a
regular file instead of a directory. When SemanticdbTaskListener.writeSemanticdb()
calls Files.createDirectories(output.getParent()), it throws FileAlreadyExistsException
(a subtype of IOException). The existing catch (IOException e) block catches it and
calls reportException() → reporter.warning() → trees.printMessage(Kind.WARNING, ...).

This approach is JDK-version-agnostic. An earlier design attempted to trigger
CompletionFailure by deleting an inner class file from the classpath, but Java 21+
handles missing inner class files silently (omitting them rather than throwing), so
that approach was not reliable across JDK versions.

Test 1 — compilation succeeds with a warning

• Blocks the output directory, compiles UsesA.java
• Asserts result.isSuccess == true
• Asserts result.stdout.contains("warning:")
• Asserts !result.stdout.contains("\nerror:")

Test 2 — semanticdb still produced for healthy files

• Compiles UsesA.java (triggers exception) alongside Healthy.java (clean)
• Asserts result.isSuccess == true
• Asserts Healthy.java has a semanticdb document in the output

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Existing tests that continue to pass

Full unit suite was run on the branch — all tests pass:

sbt "unit/test"

Key suites that exercise compilation behaviour:

• OverridesSuite — type resolution across files
• JavacClassesDirectorySuite — targetroot output
• SnapshotCommandSuite — end-to-end symbol output