`PathInfo.directory_uri` is never None

[coretl]

At the moment PathInfo.directory_uri is type hinted as being allowed to be None:

ophyd-async/src/ophyd_async/core/_providers.py

Line 27 in 624bc26

directory_uri: str | None = None

But is always calculated in post_init. If we turn this into a pydantic BaseModel then we can calculate the field on init like TriggerInfo.exposure_timeout.

Acceptance Criteria

• Converted to look like TriggerInfo and any type: ignore on calling code removed