Fix Soroban contract initialization, pure reads, and string bounds

[devfoma]

This PR resolves three critical issues regarding contract security, Soroban API interactions, and resource costs.

Changes

• Contract Initialization: Introduced an authenticated bootstrap authority for pool creation and initialization in both the repayment and pool contracts. This ensures a canonical trust boundary and prevents unauthorized takeovers.
• Pure Read Queries: Separated state-mutating keepalive operations from read-only pool queries. Pure read methods are now provided that execute without storage TTL side-effects, while legacy migrations and TTL renewals are moved to explicit, authorized keepalive operations.
• Contract String Bounds: Applied strict byte-length bounds on asset labels and reference inputs to prevent oversized inputs from causing excessive storage/event costs or exceeding Soroban resource limits.

Closes #163
Closes #179
Closes #169

[Obiajulu-gif]

Thanks for this PR! It currently has merge conflicts against main and can't be merged as-is. Could you rebase/merge main into your branch and resolve the conflicts? I'll take another look once it's conflict-free.

[Obiajulu-gif]

@devfoma resolve conflict and pr differently for the different else if will reflect in ur dashboard