chore(contracts): upgrade soroban-sdk to 26

[Fi-zzy]

Summary

• upgrade the workspace from Soroban SDK 23 to 26.1.x and align Stellar XDR on 26.0.1
• make all five contract manifests inherit SDK/XDR versions from the workspace
• align every OpenZeppelin Stellar Contracts dependency on v0.7.2
• update the Rust toolchain to 1.91.0 required by SDK 26.1.x
• update the plans test ledger fixture to protocol 26 and add the requested extern crate std

Validation

• cargo check --workspace passed
• cargo test --workspace passed: 57 tests
• all five wasm32v1-none --release builds passed
• WASM sizes: mock_pool 23,338 bytes; plans 29,054; locked_in 14,904; spend_save 11,827; target_savings 19,405 (all below 128KB)
• git diff --check passed

Closes #35

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[Fi-zzy]

Hi @Mackenzie-OO7 👋 Friendly reminder that this PR for #35 is ready for review — there are no merge conflicts and no outstanding change requests. Could you take a look and merge it when you get a chance? Happy to make any changes you'd like. Thank you!