[eye-gu]

Closes #7395.

Purpose

FlinkSourceBuilder can only produce a DataStream through build() and buildForRow(). Users who need a plain Source instance — for example to bootstrap a pipeline with historical data from a bounded Paimon source and continue streaming within one Flink HybridSource — currently have no way to obtain one.

This PR adds FlinkSourceBuilder.buildSource(), returning a Source<RowData, ?, ?> wrapped in PaimonDataStreamSource to preserve lineage (#8001). Unlike build(), no StreamExecutionEnvironment is required: boundedness is either inferred from the environment set via env() or must be given explicitly with sourceBounded(...).

build() and buildSource() share a single private construction path (createSource()), and the consumer-id validation moved into checkConsumerId() so both entry points keep it. For build() this is a pure refactor: the extra checks inside createSource() are unreachable from that path.

Only the default static and continuous scan paths are supported by buildSource(); postpone merge-on-read, scan.dedicated-split-generation, source.checkpoint-align.enabled and exactly-once consumer mode rely on other topologies and throw UnsupportedOperationException pointing to build(). The exception is postpone merge-on-read with scan.mode=compacted-full, which reads through the static path, mirroring the existing fall-through in build() (#10521).

The javadoc shows a HybridSource example and documents the pitfalls: an explicit sourceBounded(true) is needed when the bounded segment runs in a streaming job; there is no snapshot hand-off between segments, so scan.mode=latest may skip changes committed around the switch while the default latest-full re-reads and duplicates the bounded segment; and env.fromSource(...) requires an explicit TypeInformation because HybridSource is not ResultTypeQueryable.

Tests

FlinkSourceBuilderTest (26 tests): boundedness via sourceBounded(...), via env inference, missing boundedness → error; the four unsupported modes rejected; postpone merge-on-read with scan.mode=compacted-full still builds through the static path in both buildSource() and build() — locking the new checks to build()'s existing fall-through behavior.

HybridSourceITCase: a streaming job whose HybridSource embeds a bounded buildSource() segment followed by a continuous one. The continuous segment filters a <= 2, so rows 1–2 can only come from the bounded segment; DISTINCT tolerates the latest-full re-read overlap. All tests passed locally:

mvn -o -pl paimon-flink/paimon-flink-common -am -Pflink1 \
  -DfailIfNoTests=false -DwildcardSuites=none \
  '-Dtest=FlinkSourceBuilderTest,HybridSourceITCase' test