test(node): add unit tests and extract testable seams in NewBee

[martinconic]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

pkg/node/node.go (the NewBee bootstrap) had almost no test coverage, so
changes to it carried a high risk of silently breaking node startup. This PR
adds unit tests around the parts of pkg/node that can be tested in isolation,
and extracts three orchestration blocks from NewBee behind small
dependency-injection seams so their gating logic and error paths can be tested
without standing up a real chain backend, libp2p, or disk.

Validators / helpers

• validateOptions extended to cover PaymentThreshold (parse + min/max
  range), PaymentTolerance, PaymentEarly, and TargetNeighborhood.
• New validateChainContractOptions(o, chainID) covering postage / staking /
  redistribution contract-address validation, the missing postage start-block
  case, and the unknown-chain case.
• New parsePaymentThreshold shared between validation and NewBee.
• New tests for InitStateStore, InitStamperStore, overlayNonceExists,
  setOverlay, checkOverlay, InitChain (chain-disabled path), and the
  full Shutdown lifecycle.

Orchestration seams

• setupSwap, setupPostageContract, setupSwapService extracted from
  NewBee. Each takes a deps struct wired to the real constructors in
  production (defaultXxxDeps) and to fakes in tests, enabling table-driven
  coverage of every config combination and error path.

Behavior changes

• Config-shape and contract-address errors are now reported earlier in
  NewBee (at entry / right after InitChain) rather than mid-startup. The
  error strings themselves are unchanged.
• A malformed RedistributionContractAddress with EnableStorageIncentives
  disabled is still silently accepted — preserved deliberately to avoid a
  behavior change.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

Screenshots (if appropriate):

AI Disclosure

• This PR contains code that has been generated by an LLM.
• I have reviewed the AI generated code thoroughly.
• I possess the technical expertise to responsibly review the code generated in this PR.