[Contracts] errors.rs error variants lack doc comments describing when each is returned

[grantfox-oss]

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Why this matters

errors.rs defines the StreamError enum consumed by both the contract and the indexer, but individual variants have no doc comments, so callers have to grep call sites to understand when each error fires.

Acceptance criteria

• Add a doc comment above every StreamError variant explaining the triggering condition
• Cross-check each comment against actual usage in lib.rs to keep it accurate

Files to touch

• contracts/stream_contract/src/errors.rs
• contracts/stream_contract/src/lib.rs

Out of scope

Does not add, remove, or renumber error variants.