Fix publish event content type handling

[dangzitou]

Fixes #779

Summary

• infer text/plain for String, Number, and Boolean payloads when no content type is supplied
• infer application/octet-stream for byte arrays and keep their raw bytes on the gRPC path
• respect explicit content types while reusing the existing default content-type converter for recognized MIME types
• preserve custom serializers and custom MIME types

Reproduction

Before the fix, four focused regression tests were run three times. All four failed consistently: an explicit text/plain String was JSON-quoted, and implicit String, Number, and byte-array payloads were all labeled application/json.

After the fix, those cases pass, along with coverage for POJO serialization and preservation of a custom image/png byte-array payload.

Validation

• JAVA_HOME=/opt/homebrew/opt/openjdk@17/libexec/openjdk.jdk/Contents/Home ./mvnw -pl sdk clean test
• 556 tests passed; 0 failures, 0 errors, 0 skipped
• Checkstyle: 0 violations
• SpotBugs and JaCoCo completed successfully

No screenshot is needed because the regression tests assert the exact content type and wire-level protobuf payload bytes.

[dangzitou]

The current head re-triggers CI after the previous run was affected by an external example-job conflict. GitHub has marked the fork-originated workflows as awaiting maintainer approval. Could a maintainer please approve the pending Build, Auto Validate Examples, Javadocs, and FOSSA workflows? The latest commit is DCO-signed and only exists to obtain a clean validation run.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.17%. Comparing base (2edf16c) to head (8e518f4).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1797      +/-   ##
============================================
+ Coverage     77.10%   77.17%   +0.06%     
- Complexity     2317     2331      +14     
============================================
  Files           245      245              
  Lines          7186     7199      +13     
  Branches        750      758       +8     
============================================
+ Hits           5541     5556      +15     
  Misses         1284     1284              
+ Partials        361      359       -2     

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[dangzitou]

CI follow-up: I added two focused regression cases for the previously uncovered branches: implicit Boolean payloads and explicit application/cloudevents+json payloads. Local JaCoCo now reports every branch introduced by this PR as covered.

Validation:

• ./mvnw -pl sdk -Dtest=DaprClientGrpcTest test — 120 passed
• ./mvnw -pl sdk clean test — 558 passed; Checkstyle and SpotBugs passed

The remaining validate failure is in the jobs example: the Dapr scheduler reports an existing dapr-job-1 during mechanical-markdown execution. That example is outside this PR’s changed files. Could a maintainer please rerun/approve the pending workflows when convenient?