Validate object store on startup (tensorzero#1146)

* Validate object store on startup

We check that the object store is configured correctly
by writing an empty object named '.tensorzero-validate' to it.
This will help catch misconfigured S3 credentials/endpoints.

* Make test more robust

* Fix typo

* Add #[serde(default)] to e2e-only 'prefix' field

bumped version to 2025.02.5 (tensorzero#1098)

* fixed uuid7 export

* bumped version to 2025.02.5

* Fix requirements.txt

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Co-authored-by: Aaron Hill <aaron@tensorzero.com>

Bump version to 2025 02 4 (tensorzero#1087)

* bumped version to 2025.02.4

* oops

bumped version to 2025.02.3 (tensorzero#1041)

bumped version to 2025.02.2 (tensorzero#990)

bumped version to 2025.02.1 (tensorzero#965)

bumped version to 2025.02.0 (tensorzero#932)

bumped version to 2025.01.8 (tensorzero#860)

bumped version to 2025.01.7 (tensorzero#841)