[LsomeYeah]

Purpose

Fix snapshot-ID time travel when reading with a fallback or primary branch whose snapshots do not cover the requested time.

The existing rewriteOtherOptions() converts the current branch's snapshot ID to a commit time and finds a snapshot in the other branch at or before that time. When none exists, it substitutes snapshot 1 without checking availability. This makes an empty branch fail with There is currently no snapshot., can read future data when snapshot 1 exists, and fails when snapshot 1 has expired even if other snapshots remain.

Changes

• Keep snapshot selection in rewriteOtherOptions(). If no time-aligned snapshot exists, look up the earliest currently available snapshot instead of hard-coding snapshot 1.
• Add the boolean option scan.fallback-branch.allow-earliest-snapshot (default false), applying to both fallback and primary branches in this snapshot-ID alignment path, including numeric scan.version.
• For a nonempty other branch with no time match, fail by default. When the option is enabled, warn and select its earliest available snapshot, which may be newer than the requested time.
• For an empty other branch with a fixed snapshot-ID scan, warn and use a private empty scan for data planning and partition listing. The current branch is still scanned normally.
• Resolve the actual snapshot before the existing table-copy path loads its historical schema. No separate scan-time snapshot resolver or additional copyWithoutTimeTravel() call is introduced.

Compatibility

The default rejection of a nonempty branch with no time match is intentional: previously, whether future data could be read depended on whether snapshot 1 still existed. Enabling the new option explicitly permits reading the earliest available snapshot, including after snapshot 1 has expired. Empty branches are skipped regardless of the option value.

This change is scoped to the existing snapshot-ID branch-alignment path. It does not add timestamp, tag, watermark, or chain-table time-travel support. Ordinary tables still reject explicitly requested missing or expired snapshots; existing tag-backed reads are preserved. No public scan API or split-routing logic is changed.

Tests

Four focused new test methods, with 11 test/parameterized invocations, cover:

• Empty fallback/primary branches with snapshot-ID and numeric-version selectors, either option value, a latest-read control, and partition listing.
• No time-aligned snapshot with snapshot 1 present or expired, under both branch priorities: default rejection, opt-in earliest selection rather than latest, historical schema restoration after schema evolution, and option inheritance/overrides.
• Ordinary empty/expired snapshot requests remaining errors even when the option is enabled.
• Tag-backed reads succeeding when the other branch has no active snapshots.

Local validation against a817f4c75a, using Java 8:

• FallbackReadFileStoreTableTest: 37 passed.
• ChainTableFileStoreTableTest: 57 passed (existing regression tests).
• Total: 94 passed, 0 failures, 0 errors, 0 skipped.
• Core spotless:check, checkstyle:check, and git diff --check passed.

No external-engine or production integration tests were run.