Add comprehensive bridge module comments and assumptions

[Joeltabe]

closes #368

• Added module-level comments for BRIDGE_TIMEOUT_SECONDS, MAX_BRIDGE_RETRY_ATTEMPTS, and BRIDGE_RETRY_DELAY_BASE_SECONDS to explain operational rationale and units in contracts/teachlink/src/bridge.rs.
• Added function-level documentation and assumptions for mark_bridge_failed, retry_bridge, and refund_bridge_transaction explaining persisted failure indexing, exponential backoff computation (base_delay * 2^retry_count), and alias semantics.
• Replaced speculative/noisy inline commentary in add_validator with concise authorization/role-gating comments and documented assumptions for add_supported_chain, get_token, and get_admin getter behaviors.
• Inserted focused inline comments clarifying first-retry anchoring to the original transaction timestamp and the purpose of the instance-level failure index.

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[Xhristin3]

@Joeltabe resolve conflicts.

[Joeltabe]

@Joeltabe resolve conflicts.

Done