feat(invoice): complete assigned invoice and CI issues

[Oryke]

Summary:

• add CI snapshot drift detection for test snapshot files
• bound invoice metadata and payment link hash byte lengths
• use a bounded per-position merchant invoice index for pagination
• retain batch_expire input cap behavior

Closes #287
Closes #199
Closes #196
Closes #194

Validation:

• git diff --check
• cargo fmt --all: not run; cargo is unavailable in this environment
• cargo test --workspace: not run; cargo is unavailable in this environment

[drips-wave]

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