feat(exceptions): add typed exception hierarchy with machine error codes

[vitormattos]

Summary

Completes Phase 1 of the nfse-php contract layer by introducing a typed
exception hierarchy with machine-readable error codes.

New types

• NfseErrorCode — backed enum (string) with five machine error codes:
  NETWORK_FAILURE, INVALID_RESPONSE, ISSUANCE_REJECTED,
  CANCELLATION_REJECTED, QUERY_FAILED
• GatewayException — base for 4xx/5xx gateway errors; carries errorCode,
  httpStatus, and upstreamPayload (raw decoded response body for diagnostics)
• NetworkException — connectivity failures and unparseable responses
• IssuanceException extends GatewayException — gateway rejected an emit
• CancellationException extends GatewayException — gateway rejected a cancel
• QueryException extends GatewayException — gateway returned an error for a query

NfseClient changes

• request() renamed to fetchAndDecode(): now returns array{int, array}
  (HTTP status + decoded body) instead of a bare decoded array
• HTTP status is extracted from $http_response_header (set by PHP in local
  scope when using the HTTP wrapper)
• emit / query / cancel check the status and throw the appropriate typed
  exception before delegating to parseReceiptResponse
• delete() return type changed from void to array{int, array} so callers
  can inspect the cancellation status

Validation

• composer test:unit — 19/19 ✓ (6 new typed-exception tests)
• composer cs:fix — 0 files changed
• composer psalm — No errors found