fix: update rand sample_iter usage for rand 0.10 API

[ayushsingh82]

Summary

The rand crate v0.10 moved sample_iter off the Rng trait and onto Distribution — it's now called as distribution.sample_iter(rng) instead of rng.sample_iter(&distribution). This currently breaks cargo clippy and cargo test on the pending rand 0.10.0 security-update PR (#1070), in indexer-tap-agent's test.rs (confirmed via CI logs on #1070) and indexer-dips's lib.rs test module (same broken pattern, not exercised by that CI job but would break on main once #1070 merges).

This PR applies the same fix pattern used in rand's own docs (calling sample_iter on the Distribution instance) in both places, and drops the now-unused Rng import.

Changes

• crates/tap-agent/src/test.rs: generate_random_prefix() fixed
• crates/dips/src/lib.rs: VoucherContext::random() (test-only) fixed

Test plan

• cargo check -p indexer-tap-agent --tests — compiles clean
• cargo check -p indexer-dips --tests — compiles clean
• cargo fmt --check -p indexer-tap-agent -p indexer-dips — no diff
• cargo clippy -p indexer-tap-agent -p indexer-dips --all-features --all-targets -- -A dead-code -D warnings — clean, matching CI's exact invocation

This should unblock #1070 (rand 0.10.0 [security]) once rebased on top of, or merged alongside, this fix.

[ayushsingh82]

cc @MoonBoi9001 — saw you're active on tap-agent/dips lately. Quick heads up: rand 0.10 moved sample_iter from Rng to Distribution, which is currently breaking clippy/tests on the pending rand 0.10.0 security bump (#1070). This PR fixes the two broken call sites in tap-agent's test.rs and dips' lib.rs so that bump can land cleanly.