fix(client): prepare legacy graph compatibility

[imbajin]

Purpose of the PR

Legacy servers return graph configuration as JSON text, while newer APIs use Properties. Normalize that configuration for create/clone requests so the Client retains Server 1.5 compatibility. A legacy clone without configuration now sends an empty entity instead of the literal text null.

Original preparation and review: hugegraph#41.

Depends on #775. Merge the shared build preparation first. This PR targets master and currently includes prerequisite changes; after the prerequisite lands, rebase this same organization branch onto the updated upstream base and revalidate the module diff before merging.

Main Changes

• Convert supported legacy configuration values and reject null or nested values explicitly.
• Preserve create/clone request encoding and verify the real legacy HTTP route and empty clone body.
• Update task/target tests for their current cancellation and URL contracts.
• Retain the existing bytecode baseline and runtime dependency versions.

Verifying these changes

• Trivial rework / code cleanup without any test coverage.
• Already covered by Client unit tests and Checkstyle.
• Need tests and can be verified as follows:
  • JDK 11 preparation validation used hash-verified released dependencies.
  • The latest graph configuration/request regressions passed on native Java 17, including a real loopback HTTP request.
  • Current-head organization CI is still running/queued; upstream checks will run separately.

Does this PR potentially affect the following parts?

• Nope
• Dependencies (build/test tooling only; runtime payload is unchanged by this preparation)
• Modify configurations (CI/test configuration)
• The public API (compatible behavior; signatures unchanged)
• Other affects: tests and validation tooling

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need

[codecov]

Codecov Report

❌ Patch coverage is 87.87879% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.06%. Comparing base (b066b80) to head (813f155).
⚠️ Report is 246 commits behind head on master.

Files with missing lines	Patch %	Lines
...ava/org/apache/hugegraph/api/graphs/GraphsAPI.java	87.87%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master     #776       +/-   ##
=============================================
- Coverage     62.49%   46.06%   -16.44%     
- Complexity     1903     4402     +2499     
=============================================
  Files           262      602      +340     
  Lines          9541    29394    +19853     
  Branches        886     3442     +2556     
=============================================
+ Hits           5963    13539     +7576     
- Misses         3190    14551    +11361     
- Partials        388     1304      +916     

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[bitflicker64]

Blocking: yes. Summary: The JSON-to-properties conversion itself round-trips correctly through the Server 1.5 parser, but it leaves non-ASCII characters raw in a text/plain body with no charset, so on a client JVM whose default charset is not UTF-8 those config values reach the server corrupted. Score 7/10. Evidence: read the full exact-head diff and the 1.5.0 server endpoint (GraphsAPI.create consumes TEXT_PLAIN, GraphManager.cloneGraph skips an empty configText, ConfigUtil.buildConfig uses PropertiesConfiguration.read); ran the PR's encoder against commons-configuration2 2.8.0 for 17 key/value shapes (separators, comment chars, backslashes, spaces, non-ASCII), all round-trip; reproduced the charset loss with String.getBytes() under -Dfile.encoding=windows-1252 on JDK 17, where the server-side UTF-8 decode of jdbc.password=李四é yields jdbc.password=???. All latest-head checks pass except codecov/project, which reflects the stale base.