feat: Set subnet size and cost schedule of HTTP outcalls context

[eichhorl]

These fields were introduced in #10712. They are required to correctly calculate part of the new pricing inside of the execution environment, where no registry client is available.

With this PR we start setting the fields to their correct values. In a subsequent release, we can then make the cost schedule mandatory.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to b8928ba.

Security Overview

• 🔎 Scanned files: 3 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=b0761895-2609-453f-8dc1-fd7625860dc4&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/execution_environment/src/execution_environment.rs     Add cost_schedule handling when generating CanisterHttpRequestContext from flexible args and from args
Enhancement	► rs/types/types/src/canister_http.rs     Add cost_schedule parameter to CanisterHttpRequestContext constructors     Populate subnet_size and cost_schedule fields accordingly     Update tests to pass cost schedule in context generation