test(python): follow the api's unit catalogue check on timeseries create

[JosteinGj]

Summary

Follow-up to #166. I wrote that PR from probes against an api that predated platform e93271c8 and 9a8b931a. On current platform main, a unitExternalId, when sent, must name a unit in the catalogue, and it is looked up before the rest of the body is validated. test_timeseries_rejections.py now pins that:

• Stands in for unit. unit_external_id="pressure_bar" alone is created, with unit filled from the catalogue ("bar"). test(python): pin what timeseries.create refuses #166 pinned the opposite.
• Refused when blank, short or unknown. "", " ", "ab" and "no_such_unit_xyz" are each a 400 of type bad-request naming only unitExternalId. Before, the first and third were constraint-violations from the size check, and the second and fourth were accepted.
• Reported first. A bad unitExternalId is reported alone even when other fields are wrong too, because the lookup throws before validation. The several-bad-fields test now uses external_id for its third bad field.
• Disagreement kept. A unit that disagrees with the id (Celsius on pressure_bar) is stored as sent, as the platform commit says.
• The error-reading helper takes the expected problem type, and tolerates field entries without a code, which bad-request omits.

With this, #166's "things the api accepts" list reduces to the disagreeing unit, which is intended.

No SDK surface changes, so no docs pages are affected.

Test plan

• test_timeseries_rejections.py against an api built from platform main (c379d686): 29 passed
• Full Rust and Python suites on this branch merged with fix(files)!: follow the api's verbatim file external ids and deletedAt trash #164, test: run the live tests that no longer need to be skipped #165 and test(resources): use a catalogue unit external id in graph_projection_is_one_to_one_and_typed #168. The only failures left are the ClickHouse ATTEMPT_TO_READ_AFTER_EOF fault on /events/filter, and the listen test under parallel load. Neither is related.

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