Savings Goals: add remove_tags_from_goal idempotency and unknown-tag no-op tests with stale-index cleanup

[Baskarayelu]

📋 Description

SavingsGoalContract::remove_tags_from_goal in savings_goals/src/lib.rs is the inverse of add_tags_to_goal and must keep the goal's tag set and any goal-by-tag index consistent. The add path and tag-charset canonicalization are tested, but removal has gaps: removing a tag that isn't present, removing the same tag twice, and removing the last tag should all be well-defined no-ops or clean deletions.

Why this matters: tag indexes feed off-chain search and the goals-by-tag lookup. A removal path that leaves a dangling index entry (or errors on an absent tag) produces ghost search results and confuses clients. Idempotent, index-clean removal is the contract clients depend on.

🎯 Requirements & Context

Functional requirements

• Test removing a tag that is not on the goal — should be a no-op, not an error.
• Test removing the same tag twice — second call is a no-op.
• Test removing the last remaining tag leaves an empty, valid tag set.
• Assert any goal-by-tag index entry is cleaned up so the goal no longer appears under the removed tag.
• Assert owner-only authorization (Unauthorized for non-owner) and that canonicalization matches add_tags_to_goal.

Context & constraints

• Soroban SDK =21.7.7, edition 2021. Reuse the shared canonicalize_tags helper from remitwise-common/src/lib.rs.
• Test-only; do not change behavior unless a dangling-index bug is found, in which case document it.

🛠️ Suggested Execution

1. Fork & branch

git checkout -b test/savings-remove-tags-idempotency

2. Implement changes

• Add the removal tests to savings_goals/src/test.rs with /// comments per scenario.
• Document the idempotent-removal contract in docs/.

3. Test & commit

cargo test -p savings_goals remove_tags -- --nocapture

• Edge cases: mixed-case tags that canonicalize to the same value, removing tags from an archived goal, and removing a tag whose index has already been cleaned.

Example commit message

test(savings): cover remove_tags_from_goal idempotency and stale-index cleanup

Adds no-op, double-remove, last-tag, and index-cleanup assertions plus
owner-auth checks.

✅ Acceptance Criteria & Guidelines

Requirement	Target
Coverage of remove_tags_from_goal	≥ 95%
Idempotency + index-cleanup asserted	Required
Owner-auth negative test	Required
Doc comments + docs/ note	Required
cargo test -p savings_goals + cargo clippy clean	Required
Timeframe	96 hours from assignment

💬 Community & Support

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Comment when you pick this up. 🚀

[happyboy24]

CAN I WORK ON THIS PLEASE

[grantfox-oss]

🦊 GrantFox — @happyboy24 has been assigned to this issue!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #758)
2. Your PR will be reviewed by the Remitwise-Org maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@happyboy24's PR #793 was approved and merged by @Baskarayelu.

🏆 @happyboy24: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (188 total points). Track your full progress on GrantFox.

👏 Great work, @happyboy24! Keep contributing to Remitwise-Org.