feat: add _lastMessageId tracking to ObserveReadStream

[stoprocent]

Summary

This MR adds message ID tracking capability to the ObserveReadStream class, enhancing the observability of CoAP observe operations.

Changes Made

Code Changes

• Added _lastMessageId property to ObserveReadStream class in lib/observe_read_stream.ts
• Property type: number | undefined to handle cases where no messages have been processed yet
• Initialization: Set to undefined in constructor
• Updates: Property is updated in the append() method when processing observe packets

Test Coverage

• Added test in test/agent.ts within the existing "observe problems" test suite
• Test verifies:
  • Initial state of _lastMessageId
  • Proper updating after first observe notification
  • Correct tracking after subsequent notifications
• All existing tests continue to pass ✅

Technical Details

The _lastMessageId property is updated in the append() method at line 55:

this._lastMessageId = packet.messageId

This happens when a packet passes the filtering criteria and is pushed to the stream, ensuring the property always reflects the most recently processed message ID.

Benefits

• Enhanced debugging: Developers can now inspect which message ID was last processed
• Better observability: Useful for troubleshooting observe stream issues
• Consistent with existing pattern: Follows the same approach as _lastId and _lastTime properties

Testing

• ✅ New test passes successfully
• ✅ All existing observe tests continue to pass
• ✅ No breaking changes introduced
• ✅ Build process completes without errors

Files Changed

• lib/observe_read_stream.ts - Added _lastMessageId property and logic
• test/agent.ts - Added test coverage for new functionality

Breaking Changes

None. This is a purely additive change that maintains backward compatibility.

[coveralls]

Pull Request Test Coverage Report for Build 16913726014

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 92.669%

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Totals
Change from base Build 16299845999:	0.006%
Covered Lines:	2921
Relevant Lines:	3106

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 16913726014

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 92.669%

---

Totals
Change from base Build 16299845999:	0.006%
Covered Lines:	2921
Relevant Lines:	3106

---

💛 - Coveralls

[Apollon77]

I restarted the tests .. lets see if they pass now

[Apollon77]

grmppf seems we need to find out what the tests are that flaky currently :-(

@JKRhb I will be on vacation the next two weeks so no clue if I find time before mid september ... I can do reviews from my mobile, but likely no development or testing that out. But seems like the same issue already appeared with the Deps PR ...