docs(scala): fully-qualify ScalaDoc @throws and link references

[Yicong-Huang]

What changes were proposed in this PR?

Resolve the ScalaDoc broken-link warnings tracked in #4670 by replacing bare class names with fully-qualified names in @throws tags and [[X]] inline links.

• JDK exception references → java.lang.AssertionError, java.util.NoSuchElementException, java.lang.IllegalArgumentException, java.io.FileNotFoundException, java.lang.Exception, java.sql.SQLException, java.lang.reflect.AnnotatedType.
• JAX-RS exception → javax.ws.rs.BadRequestException.
• Project-internal references resolved against their companion objects: org.apache.texera.amber.core.tuple.AttributeTypeUtils.AttributeTypeException, org.apache.texera.service.resource.ComputingUnitManagingResource.DashboardWorkflowComputingUnit.
• S3StorageClient.scala — Javadoc {@link software.amazon.awssdk...} references converted to ScalaDoc [[software.amazon.awssdk...]] form (the AWS SDK references are already on the classpath; ScalaDoc was rejecting the Javadoc syntax).
• Vendored JsonSchemaInject.java — same-class method links {@link #json()} rewritten with explicit class context as {@link JsonSchemaInject#json()}.

@throws[X] annotations in code (Scala-level annotations, not docstring tags) are intentionally untouched — only the ScalaDoc * @throws X lines and [[X]] references in comments change.

Two extra docstring sites were swept up while at it (StableMergeSortOpExecSpec.scala, DatasetResource.scala) so the build does not start emitting new warnings the next time their modules' scaladoc runs.

Any related issues, documentation, discussions?

Closes #4670.

How was this PR tested?

Pure ScalaDoc / comment changes — no behavioral effect, no test changes. Verified by:

1. `grep -r '@throws AssertionError' --include='*.scala'` and the same for the other 8 bare class names returns zero hits across the repo.
2. CI's scala job runs scaladoc as part of `sbt jacoco` / compile and will report any remaining unresolved references.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.7, 1M context)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.02%. Comparing base (8841990) to head (3ed2969).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4671      +/-   ##
============================================
+ Coverage     52.46%   53.02%   +0.55%     
- Complexity        0     1994    +1994     
============================================
  Files           353      817     +464     
  Lines         21338    24630    +3292     
  Branches       1791     1921     +130     
============================================
+ Hits          11195    13060    +1865     
- Misses        10026    10914     +888     
- Partials        117      656     +539     

Flag	Coverage Δ
agent-service	?
frontend	?
python	85.05% <ø> (ø)
scala	38.17% <ø> (?)

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[aglinxinyuan]

LGTM!