fix: distinguish reflection failure from absent accessor in IcebergReflection

[unikdahal]

Which issue does this PR close?

Closes #5258.

Rationale for this change

extractFileLocation, getFileFormat, getEqualityFieldIds collapsed "accessor not
declared" and "accessor threw" into the same None/empty. Live bug: CometScanRule
reads an empty getEqualityFieldIds as "position delete" and goes native, even when
the empty was actually a swallowed failure.

What changes are included in this PR?

Option/empty now means "not declared" only; genuine invoke failures propagate.
Matches findMethod's existing contract, per review comment on the issue. No caller
changes needed at CometScanRule -- its existing outer catch just starts firing
correctly.

How are these changes tested?

New IcebergReflectionSuite cases proving failures propagate instead of collapsing.

[unikdahal]

@mbutrovich @andygrove this implements the option B you both landed on in #5258, PTAL

[sunchao]

Reviewed the distinction between absent accessors and reflection invocation failures, including the callers' fallback handling. I found no P1/P2 issues.

The exact-head ClassLoaders, IcebergReflection, and IcebergReflectionSuite sources compiled against the cached Spark 4.0.4 dependencies, and all 17 reflection tests passed locally. Exact-head CI also passed.