InstrumentSessionView should return a Visit object

Currently the `instrumentSession` provided by the component made in https://github.com/DiamondLightSource/atlas/pull/129/changes gives a string of the full session. However, it is often useful for queries to break this into proposal/sessionNumber (https://github.com/DiamondLightSource/atlas/pull/150/changes/BASE..db987190745743f998993c53c0261e472e5699dc#r3748402923). We already have a Visit object that holds the session split into parts, we should pass this around instead of the raw string

## Acceptance Criteria
* A new `fullStringRepresentation` field is added to `Visit`
* The `instrumentSession` returns a `Visit`
* Uses of the `instrumentSession` grab out of the `Visit` the data they need
* We rename `Visit` to `Session` to maintain consistency?

Start at InstrumentSessionView and trace where its instrumentSession value is created and consumed. Read the existing Visit object and its callers, then update the consumers to use the split session data and verify that instrumentSession returns a Visit with fullStringRepresentation; resolve whether the Visit-to-Session rename is in scope.

Written by the indexing model from the issue text.