feat: invoke pipeline scripts with dry-run support

[Jessicaayegh]

Added YAML/JSON invocation scripts with strict schema validation. Added sequential execution, environment interpolation, assertions, and --dry-run support. Added CI usage documentation and fixture-based tests.

Description

Adds repeatable YAML/JSON invocation scripts for sequential Soroban contract operations. Supports environment interpolation, assertions, strict schema validation, and dry-run planning for CI workflows.

Closes #821

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 change)
• Documentation update

Changes Made

• Added contract invoke-script for YAML and JSON operation scripts.
• Added sequential steps with ${VAR} environment interpolation.
• Added equals and contains return-value assertions.
• Added strict schema validation with precise errors.
• Added --dry-run to print resolved calls without RPC or submission.
• Added README, command reference, and fixture coverage.

Testing

How has this been tested?

• Added unit tests for YAML fixture parsing, interpolation, strict fields, and assertions.

• Ran editor diagnostics on all changed Rust files.

• Ran git diff --check.

• Full Cargo tests are currently blocked by unrelated existing compilation errors in other modules.

• Unit tests added/updated

• Integration tests added/updated

• Manual testing performed

Test Coverage

• Happy path: YAML parsing, sequential steps, interpolation, and dry-run planning.
• Edge cases: Unsupported schema versions, empty steps, invalid argument types, and missing environment variables.
• Error handling: Unknown fields, invalid assertions, invalid contract/network values, and missing wallets.

Code Quality Checklist

• My code follows the style guidelines of this project (cargo fmt)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings (cargo clippy -- -D warnings)
• I have added tests that prove my feature works
• New and existing unit tests pass locally with my changes
• The CI checks pass (format, clippy, tests)

Breaking Changes

• This PR introduces breaking changes

If checked, describe the breaking changes and migration path:

Not applicable.

Documentation

• README.md updated
• DEVELOPER_GUIDE.md updated (if applicable)
• API_REFERENCE.md updated (if applicable)
• No documentation changes needed

Screenshots (if applicable)

Not applicable.

Additional Context

The execute path delegates to the existing Soroban invocation logic. Dry-run validates and resolves the complete script but does not contact RPC, access wallets, or submit transactions.

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