fix: resolve 7 TypeScript compilation errors blocking tsc --noEmit

[Shantanu112-bd]

Problem

Running npx tsc --noEmit produces 7 TypeScript errors
across agent.ts, examples/, and lib/buildTransaction.ts.
This means the package cannot be safely type-checked,
breaking IDE integration and CI pipelines.

Errors

• agent.ts: invalid Server import from @stellar/stellar-sdk
• agent.ts: TS2367 non-overlapping string comparisons
• agent.ts: untyped balance inference
• examples/: implicit any in catch blocks
• lib/buildTransaction.ts: string equivalence type errors

Fix

• Correct Horizon.Server import pattern
• Add Network type union, fix comparisons
• Explicit types on balance variables
• Typed catch blocks in examples/
• Type guards in buildTransaction.ts

I have a working fix with 0 tsc errors and all 28 tests
still passing. Happy to open a PR.