test: add setupDashClient unit tests and mnemonic validation (#61)

* test: add setup dash client test cases and related dependencies

* test: use dip13KeyPath() in test helper and add setupDashClient coverage

Replace hardcoded DIP-13 path construction in deriveKeysFromMnemonic with
the exported dip13KeyPath() helper, ensuring a single source of truth for
path derivation. Add test cases for missing setupDashClient() permutations
including undefined manager guards, requireIdentity flag, and identityIndex
forwarding.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: validate mnemonic in setupDashClient before key derivation

Catch invalid BIP39 mnemonics early with a clear error message instead
of letting them propagate to the WASM SDK as opaque WasmSdkError objects.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: add dip13KeyPath unit tests for path derivation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: deepen assertions and add stubbed tests for untested paths

Verify key-role correctness (keyId per getSigner method), signer depth
(keyCount for getMaster/getFullSigner), createClient success branches
(testnet/mainnet isConnected), findNextIndex loop logic with stubbed
occupied indices, createForNewIdentity nonzero auto-index, and
AddressKeyManager getInfo/getInfoAt with stubbed SDK. Fix stale comment
and remove weak clientConfig direct-usage tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: update test scripts

Don't run setup in test:all

* chore: update mocha to latest and pin

* build: move dotenv from devDependencies to dependencies

Tutorials depend on dotenv to load .env files, so it should be a
regular dependency to ensure it is available after `npm install`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: skip mnemonic-dependent tests when PLATFORM_MNEMONIC is unset

Fork PRs don't have access to repository secrets, so identity-retrieve
fails in CI. Skip tests marked with requiresMnemonic when the env var
is absent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: thephez