fix(drive): replace unchecked integer casts with try_from

[QuantumExplorer]

Issue being fixed or feature implemented

Two unchecked integer casts could silently truncate values:

• nullifiers.len() as i64 in store_nullifiers (overflow on large counts)
• nullifiers.len() as u16 in verify_shielded_nullifiers (truncation above 65535)

What was done?

Replaced both with try_from() that returns proper errors instead of silently truncating.

How Has This Been Tested?

• cargo check -p drive — compiles cleanly

Breaking Changes

None.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

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[github-actions]

✅ DashSDKFFI.xcframework built for this PR.

• Workflow run: https://github.com/dashpay/platform/actions/runs/22760074269
• Artifacts: DashSDKFFI.xcframework (folder), DashSDKFFI.xcframework.zip, xc_checksum.txt

SwiftPM (host the zip at a stable URL, then use):

.binaryTarget(
  name: "DashSDKFFI",
  url: "https://your.cdn.example/DashSDKFFI.xcframework.zip",
  checksum: "4412837c554382cc2d071532e473315c0f0d6eb763d0843d8fa05f02324902e2"
)

Xcode manual integration:

• Download 'DashSDKFFI.xcframework' artifact from the run link above.
• Drag it into your app target (Frameworks, Libraries & Embedded Content) and set Embed & Sign.
• If using the Swift wrapper package, point its binaryTarget to the xcframework location or add the package and place the xcframework at the expected path.