Pin stellar-strkey in scaffolded templates to prevent mixed 0.0.13/0.0.16 dependency graphs

[Copilot]

Scaffolded projects were inheriting a mixed stellar-strkey graph (0.0.13 via stellar-xdr/soroban-env-* and 0.0.16 via soroban-sdk), recreating the duplicate-crate pattern previously addressed in the main workspace. This change ensures newly initialized templates explicitly align on the same stellar-strkey pin used by scaffold tooling.

• Template dependency alignment in init

  • Added a post-clone Cargo.toml patch step in stellar-scaffold init to enforce:
    • workspace.dependencies.stellar-strkey = "0.0.15"
  • Implemented via a focused helper that creates/updates [workspace.dependencies] safely.

• Fixture parity with generated template behavior

  • Updated the soroban-init-boilerplate fixture workspace manifest to include the same stellar-strkey pin, keeping test fixture behavior consistent with generated projects.

• Targeted unit coverage for manifest patching

  • Added unit tests for:
    • inserting the pin when missing
    • updating an existing outdated pin

[workspace.dependencies]
soroban-sdk = "23.4.0"
soroban-token-sdk = "23.2.1"
stellar-strkey = "0.0.15"