feat: Add merge_streams feature (#672)

[ositaemmanuel653-netizen]

Commit Type

• feat: A new feature
• fix: A bug fix
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (formatting, etc.)
• refactor: A code change that neither fixes a bug nor adds a feature
• perf: A code change that improves performance
• test: Adding missing or correcting existing tests
• build: Changes affecting the build system or external dependencies
• ci: Changes to CI/CD configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit

Breaking change? (If yes, add BREAKING CHANGE in the commit footer)

• Yes — this contains a breaking change
• No

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What changed

• Added merge_streams(stream_id_a, stream_id_b, sender) -> u64 to merge two existing streams into a single stream.
• Restricted stream merging to streams that belong to the same sender and have the same recipient.
• The merged stream's total amount is calculated from the remaining amounts of both source streams.
• The merged stream duration is based on the maximum remaining duration of the two source streams.
• Both source streams are marked as completed after a successful merge.
• Added a StreamsMerged event to record successful stream merges.
• Added/updated tests covering valid merges and validation of incompatible streams.

Testing done

• Added tests covering merging streams with the same sender and recipient.
• Verified that the merged stream contains the correct combined remaining amount.
• Verified that the merged stream uses the maximum remaining duration.
• Verified that both source streams are marked as completed after merging.
• Verified that streams with different senders or recipients cannot be merged.
• Ran the relevant test suite successfully.

Related issues

Closes #672

Checklist

• I used the correct commit type (see above) for the squashed commit message.
• I kept the change focused on the related issue.
• I added or updated tests where useful.
• I updated documentation where behavior changed.
• I verified the app still builds or explained why verification was skipped.
• My commits follow [Conventional Commits](https://www.conventionalcommits.org/) (enforced by commitlint).

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[ositaemmanuel653-netizen]

@ritik4ever