fix: save session data when session id is regenerated

[ascorbic]

Changes

Fixes a bug that caused session.regenerate() to silently lose session data

Previously, regenerated session data was not saved under the new session ID unless set() was also called. This PR fixes this by setting #dirty to true when regenrate is called.

Testing

Added a test

Docs

[changeset-bot]

🦋 Changeset detected

Latest commit: 4e3f7b8

The changes in this PR will be included in the next version bump.

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[codspeed-hq]

Merging this PR will improve performance by 10.06%

⚡ 1 improved benchmark
✅ 17 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	Build: hybrid site (static + server)	8.9 s	8.1 s	+10.06%

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_{Comparing fix/session-regenerate-dirty-flag (4e3f7b8) with main (08437d5)}