feat: add pending airdrop record unit test

[Adityarya11]

Description:

Add unit test coverage for PendingAirdropRecord, which previously had no dedicated test file.

• Add sdk/pending_airdrop_record_unit_test.go
• Add proto round-trip test covering the fungible token case
• Add String() output assertion

Related issue(s):

Fixes #1749

Notes for reviewer:

$ go test ./sdk -tags="unit" -v -run TestUnitPendingAirdropRecord -timeout 9999s
PASS

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[lfdt-bot]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 3 files with indirect coverage changes

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[Dosik13]

lgtm! thank you for the contribution!