add jspecify nullable annotations on streams Java DSL

[pjfanning]

• linked to Make use of jspecify #2563
• probably best to stick with Java DSL only
• can look at other libs later

[He-Pin]

lgtm

[He-Pin]

Good addition -- JSpecify annotations on the Java DSL will help IDE static analysis and help Java users avoid NPEs.

Observations:

1. The @Nullable annotations are applied consistently to parameters that can accept null (like rest, those, log parameters). This is correct.

2. The package-info.java with @NullMarked is the right approach -- it makes the entire Java DSL package null-safe by default, with explicit @Nullable overrides where needed.

3. JSpecify is added as an Optional dependency, which means it won't be required at runtime. This is correct since the annotations are only needed at compile time.

4. One thing to consider: this PR only covers the stream Java DSL. The PR description mentions 'probably best to stick with Java DSL only' -- but the actor and other Java DSLs would benefit from the same treatment. Consider filing a follow-up issue for the remaining modules.

No blocking issues.

[pjfanning]

@He-Pin if we don't see any issues caused by this, I'll look into adding similar changes to other modules.