Fix #427: preserve original event timestamp in Event.forward

[wbarnha]

What

Event.forward re-sends the original message ("will not be reserialized"), and already defaults key, value and headers to the original message via USE_EXISTING_* sentinels. But timestamp defaulted to None, so the forwarded message received a fresh send-time timestamp instead of the original event time.

Stream.group_by forwards events internally, so grouping silently dropped the event timestamp — breaking downstream event-time windowing (the user has to rebuild the partitioning key by hand to keep the timestamp).

Closes #427.

How

• Added a USE_EXISTING_TIMESTAMP sentinel alongside the existing USE_EXISTING_KEY / USE_EXISTING_VALUE / USE_EXISTING_HEADERS.
• Event.forward now defaults timestamp to self.message.timestamp, consistent with how it already treats the other forwarded fields.

message.timestamp is stored in seconds (the consumer divides the Kafka record's millisecond timestamp by 1000), which is the same unit send() expects (it multiplies back by 1000), so the value round-trips correctly. Passing an explicit timestamp — including None — still overrides, so callers keep full control.

Test

Added test_forward__default_timestamp_preserved in tests/unit/test_events.py: with no timestamp argument, asserts forward passes self.message.timestamp through to _send. The existing test_forward (explicit timestamp) and test_forward__USE_EXISTING_KEY_VALUE (explicit timestamp=None) still pass unchanged, confirming explicit values are respected. Full tests/unit/test_events.py passes (9 passed); events + group_by/forward stream tests green (17 passed).

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (3073eb9) to head (a557385).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #705      +/-   ##
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+ Coverage   94.14%   94.15%   +0.01%     
==========================================
  Files         104      104              
  Lines       11136    11139       +3     
  Branches     1201     1202       +1     
==========================================
+ Hits        10484    10488       +4     
+ Misses        551      550       -1     
  Partials      101      101              

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