[Enhancement] Support multiple simultaneous MockToken registrations via with_tokens() builder method

[ekelemepraise-code]

Summary

Setting up several tokens in a fixture requires repeated .with_token() calls:

let env = MockEnv::builder()
    .with_token("USDC", 6)
    .with_token("EURC", 6)
    .with_token("BTC",  8)
    .with_token("ETH",  18)
    .build();

A bulk registration method would reduce noise in fixture setup code.

Proposed API

let env = MockEnv::builder()
    .with_tokens([
        ("USDC", 6),
        ("EURC", 6),
        ("BTC",  8),
        ("ETH",  18),
    ])
    .build();

Or using a typed enum for well-known assets:

.with_tokens([WellKnownAsset::USDC, WellKnownAsset::EURC])

Acceptance Criteria

• with_tokens() accepts any IntoIterator<Item = (&str, u32)>
• All tokens accessible via env.token("USDC") after build
• WellKnownAsset enum added with USDC, EURC, XLM variants (optional, feature-gated)
• Existing with_token() unchanged