fix(sessions): preserve Dapr session created_at across writes

[adityasingh2400]

Summary

DaprSession.add_items() rewrote the session metadata on every call, clobbering created_at with the current timestamp on each write. The field now reflects the true session start time, while updated_at advances as expected.

• Read back the existing metadata before writing and reuse the prior created_at if it parses as a dict with that field.
• Fall back to the current timestamp only when metadata is missing or corrupt (handled via guarded JSON / decode error catch).
• Drop a duplicated etag = getattr(response, "etag", None) or None line in pop_item() (dead code).

Test plan

• New test_metadata_preserves_created_at asserts created_at is stable across two add_items calls one second apart while updated_at advances.
• Full tests/extensions/memory/ suite passes (180 passed, 5 skipped).
• ruff check and ruff format --check clean on touched files.

[seratch]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Guard metadata update with an ETag

This fix reads existing metadata and then overwrites the metadata key without any ETag/concurrency option. With the default eventual reads or two DaprSession instances writing the same session, a stale/missing metadata read leaves created_at = now, and this unconditional save can clobber an older created_at, so the field can still advance across writes despite the commit intent.

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