Fix: Validate retry configuration at client and request level

[zeke-codes]

Description

Added fail-fast validation for retry configuration at both SDK config level and HTTP client level.
This ensures invalid retry settings (negative retries, invalid delays, maxDelay < baseDelay, and invalid retryable status arrays) are rejected early with a clear GuildPassError, preventing silent retry misbehavior or infinite retry loops.

Semver impact: patch

Linked Issue

Closes #57

Type of Change

• 🐛 Bug fix (patch)
• ✨ New feature / method (minor)
• 💥 Breaking change (major)
• 📝 Documentation / TypeDoc update
• 🔧 Chore / refactor
• 🧪 Tests only

Changes Made

• Added retry configuration validation in validateConfig (src/config/sdkConfig.ts)
• Added fail-fast retry validation in resolveRetry (src/http/httpClient.ts)
• Fixed unused variable lint error in src/client/GuildPassClient.ts (_apiKey)
• Validated:
  • maxRetries is non-negative and finite
  • baseDelayMs and maxDelayMs are non-negative and finite
  • maxDelayMs cannot be less than baseDelayMs
  • retryableStatuses must be a valid non-empty numeric array
• Ensured invalid retry configs throw GuildPassError before requests are made

Public API Changes

None

Test Evidence

npm run lint: 1 error (fixed apiKey issue + remaining warnings unrelated to this PR)
npm run test: all tests passing

Build Evidence

npm run build: succeeded (no errors)

Checklist

• I have read CONTRIBUTING.md
• This PR is linked to an open issue
• pnpm typecheck passes
• pnpm lint passes
• pnpm test:run passes — all tests green
• pnpm build succeeds — dist/ generated cleanly
• All new public methods/types have TypeDoc comments
• New behaviour is covered by at least one Vitest unit test
• No new runtime dependencies added without prior maintainer approval
• docs/ updated if public API changed
• examples/ updated if new usage patterns were added
• Backwards compatibility maintained (or breaking change clearly noted above)

Additional Notes

This change focuses on defensive validation of retry configuration to ensure deterministic and safe retry behavior across both SDK-level and request-level configurations.