[client] Introduce dedicated client.lookup.close-timeout config option

[XuQianJin-Stars]

Purpose

LookupClient#close(Duration timeout) already accepts a Duration parameter,
but the only caller in FlussConnection#close() was hard-coding
Duration.ofMillis(Long.MAX_VALUE) to let pending lookup requests drain.
That hard-coded value is undesirable in K8s / graceful-shutdown scenarios
where the client must finish within a bounded budget (e.g. before SIGTERM
SIGKILL grace period elapses), and it is also harder to tune per-deployment.

This PR introduces a dedicated, user-tunable config option
client.lookup.close-timeout and wires it through the only existing caller,
while keeping the public LookupClient#close(Duration) signature intact so
that no downstream code is broken.

Linked issue: N/A (usability / K8s shutdown observability improvement)

Brief change log

• Add new ConfigOption<Duration> CLIENT_LOOKUP_CLOSE_TIMEOUT
  (client.lookup.close-timeout) in
  fluss-common/src/main/java/org/apache/fluss/config/ConfigOptions.java,
  with default value Long.MAX_VALUE (preserves previous behavior) and a
  javadoc-style description explaining the K8s / SIGTERM use case.
• Update FlussConnection#close() in
  fluss-client/src/main/java/org/apache/fluss/client/FlussConnection.java:
  read the new option from the connection's Configuration and pass it to
  lookupClient.close(...) instead of the hard-coded
  Duration.ofMillis(Long.MAX_VALUE). The previous comment is updated to
  reflect the new behavior.
• No change to LookupClient#close(Duration) itself — its public
  signature is preserved, so existing direct callers keep working.
• Add new test class
  fluss-client/src/test/java/org/apache/fluss/client/lookup/LookupClientCloseTest.java
  covering 5 close-timeout scenarios (see Tests).

Tests

• LookupClientCloseTest#testCloseWithVeryShortTimeoutReturnsNormally — closes
  a fresh LookupClient with Duration.ofMillis(1); asserts no exception is
  thrown and no InterruptedException leaks.
• LookupClientCloseTest#testCloseWithZeroTimeoutReturnsNormally — closes
  with Duration.ZERO; verifies the API tolerates a zero-duration close.
• LookupClientCloseTest#testCloseWithLargeTimeoutReturnsNormally — closes
  with Duration.ofSeconds(5); sanity check for the normal happy-path
  duration.
• LookupClientCloseTest#testCloseIsIdempotent — closes the same client
  twice in a row; verifies the second call also returns without error.
• LookupClientCloseTest#testDefaultCloseTimeoutConfigValue — asserts the
  default value of CLIENT_LOOKUP_CLOSE_TIMEOUT is
  Duration.ofMillis(Long.MAX_VALUE), locking in backwards compatibility.

All 5 tests are unit tests (no real cluster required); they build a
LookupClient via a TestingMetadataUpdater. Pre-existing
fluss-client tests are unaffected — no regression observed locally.

API and Format

• No public API removed or renamed. LookupClient#close(Duration) keeps
  the same signature.
• New config option follows the
  AGENTS.md §8 Configuration Patterns: durationType(),
  explicit defaultValue(...), javadoc withDescription(...),
  hierarchical key client.lookup.close-timeout.
• mvn spotless:apply clean.
• Java 8 compatible: no var, no Optional.isEmpty, no List.of.
• All test assertions use AssertJ (assertThat(...).isEqualTo(...) /
  assertThatCode(...).doesNotThrowAnyException()).
• Commit title [client] Introduce dedicated client.lookup.close-timeout config option follows the <component> prefix convention.
• ./mvnw clean install -DskipTests -T 32 passes locally on JDK
  11.0.23-kona.

Documentation

• The new option's javadoc / withDescription already documents the K8s
  SIGTERM use case, so it will be picked up by the doc generator
  (./mvnw -pl fluss-docgen verify) automatically.
• No changes to the public website are needed beyond the auto-generated
  config reference entry.