[improve][client] Support protobuf v4 schema compatibility

[codelipenghui]

Motivation

Fixes #25202.

Pulsar protobuf schema APIs were bounded to GeneratedMessageV3, which breaks usage with protobuf v4 generated classes (for example Any).

Modifications

• Widened protobuf schema-related generic bounds from GeneratedMessageV3 to Message in client API/binding and implementations.
• Updated protobuf schema reader/writer/native classes and runtime validation checks to use Message.
• Kept strict generated-message validation via descriptor/default-instance reflective paths.
• Added API signature regression test to lock in Message upper bounds.
• Added/updated protobuf schema tests for client and native paths.
• Updated pulsar-functions protobuf class detection to work with protobuf v4 (Message-based detection).
• Added protobuf-v4 required CI job for client + functions tests using v4 property overrides.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[github-actions]

@codelipenghui Please add the following content to your PR description and select a checkbox:

- [ ] `doc` <!-- Your PR contains doc changes -->
- [ ] `doc-required` <!-- Your PR changes impact docs and you will update later -->
- [ ] `doc-not-needed` <!-- Your PR changes do not impact docs -->
- [ ] `doc-complete` <!-- Docs have been already added -->

[codecov-commenter]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.61%. Comparing base (82846e5) to head (d04cbd4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...ulsar/client/impl/schema/ProtobufNativeSchema.java	33.33%	1 Missing and 1 partial ⚠️
...ache/pulsar/client/impl/schema/ProtobufSchema.java	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #25261      +/-   ##
============================================
- Coverage     72.73%   72.61%   -0.13%     
- Complexity    34047    34413     +366     
============================================
  Files          1959     1959              
  Lines        155418   155418              
  Branches      17731    17731              
============================================
- Hits         113050   112855     -195     
- Misses        33378    33562     +184     
- Partials       8990     9001      +11     

Flag	Coverage Δ
inttests	25.56% <0.00%> (-0.60%)	⬇️
systests	22.38% <0.00%> (-0.08%)	⬇️
unittests	73.59% <55.55%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...main/java/org/apache/pulsar/client/api/Schema.java	83.33% <ø> (+3.33%)	⬆️
...nt/internal/PulsarClientImplementationBinding.java	62.50% <ø> (ø)
...nt/impl/PulsarClientImplementationBindingImpl.java	81.81% <ø> (+1.51%)	⬆️
...lient/impl/schema/reader/ProtobufNativeReader.java	100.00% <ø> (ø)
...lsar/client/impl/schema/reader/ProtobufReader.java	57.14% <ø> (ø)
...lient/impl/schema/writer/ProtobufNativeWriter.java	100.00% <ø> (ø)
...lsar/client/impl/schema/writer/ProtobufWriter.java	100.00% <100.00%> (ø)
...ulsar/functions/instance/JavaInstanceRunnable.java	73.71% <100.00%> (+0.53%)	⬆️
...rg/apache/pulsar/functions/source/TopicSchema.java	70.88% <100.00%> (+2.53%)	⬆️
...ulsar/client/impl/schema/ProtobufNativeSchema.java	79.06% <33.33%> (ø)
... and 1 more

... and 106 files with indirect coverage changes

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[lhotari]

LGTM. I revisited the CI build to use the standard way for unit tests.