Fully enable NullAway:JSpecifyMode checking

[schlosna]

Before this PR

We were not using the full JSpecify mode of NullAway and several packages were not fully @NullMarked.

After this PR

==COMMIT_MSG== All tritium packages are fully @NullMarked with jspecify @Nullable or @NonNull annotations as needed by APIs. Enable JSpecify mode for stricter NullAway nullness checking.

See https://github.com/uber/NullAway/wiki/JSpecify-Support and https://github.com/uber/NullAway/wiki/How-NullAway-Works#jspecify ==COMMIT_MSG==

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• [ ] Fix (Fixing an issue with existing functionality)
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• [ ] Deprecation (Removing functionality in a non-breaking way)
• [ ] Migration (Automatically moving data/functionality to a new system)

Description All tritium packages are fully @NullMarked with jspecify @Nullable or @NonNull annotations as needed by APIs. Enable JSpecify mode for stricter NullAway nullness checking.

See https://github.com/uber/NullAway/wiki/JSpecify-Support and https://github.com/uber/NullAway/wiki/How-NullAway-Works#jspecify

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