fix/test/docs: negative interval accumulation with incentive rewards

[p0mvn]

Progress towards: #5854

What is the purpose of the change

Please review #5869 first for better context and easier review experience.

This PR fixes a negative interval accumulation edge case bug by simply replacing Sub with SafeSub where applicable. It introduces new coming helpers in osmoutils to achieve that.

The tests are extended to cover claiming rewards below, inside, and above the position after it is initialized.

Note, I noticed another edge case around negative values while working on this. Added docs to explain it. It is covered by current test

Testing and Verifying

The global invariants are asserted, and the tests ensure that the appropriate amount is claimed from all positions in all cases.

Documentation and Release Note

• Does this pull request introduce a new feature or user-facing behavior changes?
• Changelog entry added to Unreleased section of CHANGELOG.md?

Where is the change documented?

• Specification (x/{module}/README.md)
• Osmosis documentation site
• Code comments?
• N/A

[czarcas7ic]

Needs second ACK due to change with incentives.go, but LGTM

[AlpinYukseloglu]

Core changes LGTM! Did slightly more cursory review of tests but they generally look good (will try to do another pass later to make sure the changes are covered but shouldn't be blocking)

[ValarDragon]

I think theres some merge conflict issues with the other PR, PR LGTM assuming these are resolved