refactor(starfish): replace string-based source tracking with TransactionSource enum

[piotrm50]

Description of change

Refactored transaction source tracking in the Starfish consensus implementation to use a type-safe enum instead of string literals for better maintainability and type safety.

Changes:

• Created TransactionSource enum in dag_state.rs with variants: TransactionSynchronizer, BlockStreaming, ShardReconstructor, and Test
• Updated DagState::add_transactions() to accept TransactionSource instead of &str
• Updated all call sites to use enum variants instead of string literals

Benefits:

• Type safety: Compiler enforces valid source values
• Better organization: Enum colocated with DagState where it's primarily used
• Cleaner API: Self-documenting enum variants replace magic strings
• Improved maintainability: Easier to track all transaction sources

Links to any relevant issues

Fixes #8774

How the change has been tested

• Basic tests (linting, compilation, formatting, unit/integration tests)
• Patch-specific tests (correctness, functionality coverage)
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

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6 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
apps-backend	Ignored	Preview		Oct 23, 2025 11:37am
apps-ui-kit	Ignored	Preview	Comment	Oct 23, 2025 11:37am
iota-evm-bridge	Ignored	Preview		Oct 23, 2025 11:37am
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