perf: remove duplicated check of same key in the instant send database

[knst]

Issue being fixed or feature implemented

Previous implementation checked both Deterministic and Non-deterministic instant send locks; though once non-determinstic has been removed, implementation has been updated and now the same key is checked twice.

What was done?

This PR removes duplicated check

How Has This Been Tested?

See CI

Breaking Changes

N/A

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 made corresponding changes to the documentation
• I have assigned this pull request to a milestone

[github-actions]

✅ No Merge Conflicts Detected

This PR currently has no conflicts with other open PRs.

[coderabbitai]

Walkthrough

The change modifies GetInstantSendLockByHashInternal in src/instantsend/db.cpp to optimize hash lookup logic. When a hash is missing or mismatched, the function now early-returns with a nullptr instead of performing a second redundant read operation and constructing an extra object. The caching behavior and overall function semantics remain unchanged.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

• Key area for attention: Verify that the early-return logic correctly handles all edge cases (missing hash, mismatched hash) and doesn't unintentionally skip any validation or caching steps that the previous two-read pattern relied on
• Secondary focus: Confirm that callers of this internal function can safely handle nullptr returns and that no assumptions about object construction timing are violated

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main optimization: removing a duplicated key check in the instant send database, which aligns with the file changes and PR objectives.
Description check	✅ Passed	The PR description clearly relates to the changeset: it describes removing a duplicated check of the same key in instant send database, which aligns with the code changes removing redundant logic in GetInstantSendLockByHashInternal.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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