Wire tier-1 typed-event conformance fixtures

Move 10 LlmCompletionEvent/LlmFailedEvent fixtures (060-065, 067, 068,
071, 072) from _UNIT_TESTED_FIXTURES into _SUPPORTED_FIXTURES so the
conformance harness runs the spec's own YAML fixtures rather than
python's hand-written equivalents. First tier of the fixture-harness
catch-up; strengthens the cross-impl conformance signal with no library
change.

Four of the family stay unit-tested, each blocked on a spec-side fixture
change to be picked up at the next pin bump: 066 (single-member prompt
group, corrected upstream), 069 (asserts an undeclared request model),
070 (missing tool_call_id is non-constructible here, validated at
construction not the call boundary), 073 (asserts the vendor error.type
where python surfaces the exception class name).

Author: chris-colinsky