Add RestSessionCatalog

[DerGut]

Which issue does this PR close?

• This is the second of ~3 PRs toward Session/ Request/ Auth Context Propagation #2774.
• Depends on Add SessionCatalog trait #2836.

What changes are included in this PR?

This PR adds RestSessionCatalog, implementing the SessionCatalog API introduced in #2836.

RestSessionCatalog becomes the core REST implementation. The existing RestCatalog delegates to it with an internal SessionContext, preserving session-unaware behavior. The REST test suite is refactored to exercise RestSessionCatalog directly.

Welcome Feedback ❓

The main question I had while implementing this is which canonical variable name we should introduce for the SessionContext. The current implementation uses session because there's already a context in the RestSessionCatalog's scope (which deals with the lazy creation of the client via the /v1/config endpoint).

Arguably, the SessionContext isn't equivalent to a session. It may carry state from a query engine's session, and it may be converted to an AuthSession which is a session in the Iceberg sense. But maybe it's close enough.

An alternative way to name the SessionContext in variables would be to rename the RestContext type to something like RestClient, we'd free the concept of a context in that scope.

Follow-ups

This PR is going to be followed by a integrations/datafusion PR that starts using a SessionCatalog to propagate Datafusion sessions.

Are these changes tested?

The existing test suite passes after backing the RestCatalog with the RestSessionCatalog (first commit c7fd2fe). Afterwards, I've refactored the test suite to test the RestSessionCatalog directly (as it now contains the core logic), and added some additional tests for session catalog creation and integration-style tests that assert the delegation is working.

[DerGut]

As mentioned in the "Welcome Feedback ❓" section, I could use some opinions on the session vs. context/ ctx variable naming. I feel like what we introduce here will be reused throughout.

Suggested change

	pub fn with_session(mut self, session: SessionContext) -> Self {
	pub fn with_session_context(mut self, ctx: SessionContext) -> Self {