feat: accept empty content body in HttpJsonTranscodingService

[DongHyukki]

Motivation

HttpJsonTranscodingService currently fails with 400 Bad Request when handling a request with Content-Type: application/json and no body, even if the corresponding google.api.http rule uses body: "*". This behavior is inconsistent with common implementations such as Envoy and grpc-gateway, which treat empty bodies as {} in such cases.

⸻

Modifications

• HttpJsonTranscodingService
  • Added a fallback to use an empty JSON object when body empty ( content-length: 0 )
  • Treat empty request bodies as {} when all of the following are true:
    • Content-Type: application/json
    • google.api.http rule: body: "*"

⸻

Result

• Closes HttpJsonTranscodingService throws on empty DELETE body due to gRPC HttpRule mapping #6319.
• Users can now send requests with Content-Type: application/json and an empty body without receiving a 400 Bad Request error.
• Ensures better compatibility with gRPC transcoding conventions from other ecosystems

[DongHyukki]

@minwoox
Hi! This PR fixes #6319
When you have a moment, could you please take a look? Thank you!

[github-actions]

🔍 Build Scan® (commit: b90e9d7)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/2vosgw7uso6dg
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/e3sorthh7y5zu
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/3ymmtagfjhmxe
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/clamdnsbumxlu
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/zuxj2k7b5zcgy
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/d774vhdpkpb5o
build-macos-latest-jdk-21	❌ (failure)	https://ge.armeria.dev/s/p2dwnu5yzey6o

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (8150425) to head (b90e9d7).
⚠️ Report is 130 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #6325       +/-   ##
============================================
- Coverage     74.46%       0   -74.47%     
============================================
  Files          1963       0     -1963     
  Lines         82437       0    -82437     
  Branches      10764       0    -10764     
============================================
- Hits          61385       0    -61385     
+ Misses        15918       0    -15918     
+ Partials       5134       0     -5134     

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[ikhoon]

Thanks!

[minwoox]

Thanks! 👍