Migrate to JSpecify annotations

[sh-cho]

Motivation:
jsr305 is dormant and now JSpecify is recommended annotation sets for variety of static analyses (for now, it supports only nullness) and many open sources are moving to JSpecify (e.g. Spring Framework, Guava, ...)

• https://jspecify.dev/
• https://spring.io/blog/2025/03/10/null-safety-in-spring-apps-with-jspecify-and-null-away

Modification:

• Replace javax.annotation.Nullable, com.linecorp.armeria.common.annotation.Nullable with org.jspecify.annotations.Nullable
• Use @NullMarked (org.jspecify.annotations.NullMarked) instead of NonNullByDefault and apply it to package-info.javas
• Use @Nullable within inner or fully-qualified type names (Java enforces this to @Target(ElementType.TYPE_USE))
  • as-is: @Nullable public static com.linecorp.centraldogma.common.EntryType convertEntryType(...)
  • to-be: public static com.linecorp.centraldogma.common.@Nullable EntryType convertEntryType(...)
  • (ref: https://docs.oracle.com/javase/specs/jls/se19/html/jls-9.html#jls-9.7.4)

Result:
resolves #1193

Note:
For armeria service, I didn't change existing nullability annotations which used as parameter because it breaks tests.
(ex. ContentServiceV1) -> JsonPatchOperationIntegrationTest failed
I debugged shortly and looks like JSpecify`s nullability annotation is not handled correctly in armeria. It can be handled in other proj / issue.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.77%. Comparing base (ca4e9d7) to head (c8cb731).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1194      +/-   ##
============================================
- Coverage     70.68%   68.77%   -1.92%     
+ Complexity     4764     4668      -96     
============================================
  Files           475      480       +5     
  Lines         19633    20109     +476     
  Branches       2158     2249      +91     
============================================
- Hits          13877    13829      -48     
- Misses         4583     5074     +491     
- Partials       1173     1206      +33     

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[minwoox]

@sh-cho Thanks for the PR! 👍
I want to apply this to Armeria first.
Would you check if the annotation works correctly, in the situation that we mentioned in this issue, please?
line/armeria#2793

[sh-cho]

@minwoox Ok let me check armeria first