fix: Do not charge pay-as-you-go base fee on system subnets

[eichhorl]

System subnets have a normal cost schedule, but HTTP outcalls are still free.

This was already correctly enforced for the per-replica and consensus fees under the new pay-as-you-go pricing.

With this PR we also enforce a free base fee on system subnets.

Additionally, similar to (non-)replicated outcalls, we disable flexible outcalls on subnets where the http_requests feature is disabled.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 702eca7.

Security Overview

• 🔎 Scanned files: 4 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=f95e4d84-4dac-46da-a446-673cdc975cfb&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/embedders/src/wasmtime_embedder/system_api.rs     Cost schedule handling: free outcalls on system subnets implemented plus adjustment to subnet_cycles_config
Enhancement	► rs/execution_environment/src/execution_environment.rs     Subnets: system subnets have Free cost schedule for HTTP outcalls; pricing version logic updated based on subnet type and http_requests feature; adjust pricing calculation and available options for flexible outcalls
Enhancement	► rs/execution_environment/src/execution_environment.rs     Update to use cost_schedule from context and align cycles config for HTTP outcalls
Enhancement	► rs/execution_environment/src/execution_environment.rs     Remove http_outcalls_are_free helper and inline free check against cost_schedule where used
Enhancement	► rs/execution_environment/src/execution_environment/tests.rs     Add tests around free outcalls on system subnet and free cost schedule, including various pricing versions for HTTP requests and flexible requests
Enhancement	► rs/execution_environment/tests/hypervisor.rs     Add tests verifying cost_http_request_v2 is free on system subnet and on free cost schedule
Enhancement	► rs/execution_environment/tests/hypervisor.rs     New test helper cost_http_request_v2_is_free_on and related tests for free scenarios