fix(spec): accept the legacy first_not_null_value aggregate name

[jackylee-ch]

Java registers first_not_null_value as an SPI alias of first_non_null_value:
FieldFirstNonNullValueAggLegacyFactory sits next to the canonical factory in
META-INF/services/org.apache.paimon.factories.Factory and builds the same
aggregator. It is deliberate rather than residue — it survives the switch-case to
SPI refactor in #4335 and FirstValueAggregationITCase still creates a table with
it — but undocumented, so nothing advertises it.

Rust rejected the name at all three sites that key off it, so a table written by
Java or Flink planned fine and then failed once rows were pulled, with
ConfigInvalid: Unknown aggregate function 'first_not_null_value'. CREATE and
ALTER rejected it outright. The write path never checks the name, which is why
this only surfaces where the merge function is built.

Resolved through a single canonicalization point rather than one arm per site: the
validator/constructor guard test iterates a hardcoded name list, so a one-sided
edit would slip past it. Matching is exact, mirroring FactoryUtil#discoverFactory,
which compares identifiers with equals.

The alias stays out of the supported: ... hint — Java does not document it, so it
is read but never suggested — and errors still echo whatever the user wrote.

FieldAggregator::name is &'static str, so the aggregator reports the canonical
name. That costs no parity: the only Java message repeating the configured
identifier is the retract rejection, and retract is refused here before an
aggregator exists.

[QuakeWang]

LGTM