docs: clarify private key types and usage across examples

#353 (predicates epic) must be fully complete before starting this task.

JS reference: [PR #1000](https://github.com/hashgraph/hedera-agent-kit-js/pull/1000)

## Scope

- Fix `from_ed22519()` typo → `from_ed25519()` in all `.env.example` comments
- Align READMEs to cover both ECDSA and ED25519 key types
- Explain `PrivateKey.from_string()` auto-detection vs explicit `from_ecdsa()` / `from_ed25519()`
- Standardize across `langchain/`, `adk/`, and `langchain-classic/` example directories

Wait until #353, the predicates epic, is complete. Review PR #1000 for the JS reference, then inspect the .env.example comments and READMEs under langchain/, adk/, and langchain-classic/. Done means correcting every from_ed22519() typo and documenting ECDSA and ED25519 keys, including auto-detection with PrivateKey.from_string() and explicit constructors.

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