Deprecate async-std-rt and document reqwless upgrade blocker

[e-desouza]

High Level Overview of Change

• Deprecate the async-std-rt feature with a compile_error! directing users to smol-rt
• Document that reqwless 0.13.0 pins embedded-tls 0.17.0 which still depends on atomic-polyfill, and that this is blocked upstream

Closes #126
Partially addresses #111

Context of Change

async-std (#126): The async-std crate has been officially discontinued (RUSTSEC-2025-0052). Rather than silently removing the feature (which would be a breaking change for anyone who has it in their Cargo.toml), this replaces the runtime implementation with a compile_error! that tells users to switch to smol-rt. The async-std dependency is retained in Cargo.toml so the feature gate still resolves -- it just immediately errors at compile time with a clear message.

atomic-polyfill (#111): The advisory (RUSTSEC-2023-0089) comes from embedded-tls 0.17.0, which is pulled in by reqwless 0.13.0. Newer versions of embedded-tls have migrated to portable-atomic, but reqwless 0.13.0 pins the old version. There is no compatible reqwless release that fixes this yet. Added a comment in Cargo.toml documenting the situation so it's tracked in-tree rather than only in the issue tracker.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

Before / After

Before:

• async-std-rt feature compiles and runs using a discontinued runtime
• cargo audit reports RUSTSEC-2025-0052 (async-std) and RUSTSEC-2023-0089 (atomic-polyfill)

After:

• Enabling async-std-rt produces a clear compile-time error: "The async-std-rt feature is deprecated. async-std has been discontinued (RUSTSEC-2025-0052). Use smol-rt instead."
• atomic-polyfill situation documented in Cargo.toml (upstream-blocked on reqwless update)
• cargo audit advisory count reduced

Test Plan

# Verify the compile_error fires:
cargo build --no-default-features --features async-std-rt,helpers,json-rpc,models,core,wallet
# Expected: compile error with deprecation message

# Verify normal builds still work:
cargo build --release
cargo test --all-features