[server] Support configurable time partition format for auto-partitioned tables

[wattt3]

Purpose

Linked issue: close #3191

Brief change log

• Add table.auto-partition.time-format to override the unit partition value format
• Validate pattern syntax at table creation
  • the zero-padded / lex-orderable contract is the users's responsibility. (retention compares partition names via TreeMap)

Tests

API and Format

Documentation

[wattt3]

@luoyuxia PTAL, thanks.

[Copilot]

Pull request overview

Adds a new table option to customize the string format used for time-based auto-partition values, propagating that option through partition generation/retention logic and documenting the new behavior.

Changes:

• Introduce table.auto-partition.time-format (no default; derived from time-unit when unset) and wire it into auto-partition creation and retention.
• Validate custom time format pattern syntax during table descriptor validation, with new unit tests.
• Update docs and existing tests/call sites for updated partition utility APIs.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
website/docs/table-design/data-distribution/partitioning.md	Documents the new table.auto-partition.time-format option for auto-partitioned tables.
website/docs/engine-flink/options.md	Exposes the new option in Flink engine table options documentation.
fluss-server/src/test/java/org/apache/fluss/server/utils/TableDescriptorValidationTest.java	Adds coverage for accepting/rejecting custom time-format values at table creation validation time.
fluss-server/src/test/java/org/apache/fluss/server/coordinator/TableManagerITCase.java	Updates test helper call site for new generateAutoPartition(...) signature.
fluss-server/src/main/java/org/apache/fluss/server/utils/TableDescriptorValidation.java	Adds table-create-time validation for time-format pattern syntax.
fluss-server/src/main/java/org/apache/fluss/server/coordinator/AutoPartitionManager.java	Passes time-format through to partition pre-creation and retention cutoff calculation.
fluss-common/src/test/java/org/apache/fluss/utils/PartitionUtilsTest.java	Adds tests for partition name generation with a custom time format.
fluss-common/src/main/java/org/apache/fluss/utils/PartitionUtils.java	Extends partition time generation/validation to accept an optional custom time-format and uses Locale.ROOT.
fluss-common/src/main/java/org/apache/fluss/utils/AutoPartitionStrategy.java	Adds timeFormat to the resolved auto-partition strategy from table options.
fluss-common/src/main/java/org/apache/fluss/config/ConfigOptions.java	Defines the new table.auto-partition.time-format config option and its description.

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[wattt3]

@luoyuxia
Addresed Copilot's review.
PTAL, thanks.