fix: replace generic exception handling

[tcheeric]

Summary

Related issue: #____

What changed?

• Introduced dedicated Schnorr and Bech32 exception types and updated callers to eliminate catch (Exception) blocks in signing and key encoding flows.
• Hardened Identity and NostrSpringWebSocketClient error handling to surface explicit failures instead of wrapping unknown exceptions.
• Updated unit tests to reflect the new exception contracts and documented their intent per repository guidelines.

BREAKING

None.

Review focus

• Validate that the new exception hierarchy aligns with expectations for consumers of signing/encoding APIs.

Checklist

• Scope ≤ 300 lines (or split/stack)
• Title is verb + object (e.g., “Refactor auth middleware to async”)
• Description links the issue and answers “why now?”
• BREAKING flagged if needed
• Tests/docs updated (if relevant)

$ mvn -q verify
[ERROR] [ERROR] Some problems were encountered while processing the POMs:
[ERROR] Non-resolvable import POM: The following artifacts could not be resolved: xyz.tcheeric:nostr-java-bom:pom:1.1.1 (absent): Could not find artifact xyz.tcheeric:nostr-java-bom:pom:1.1.1 in central (https://repo.maven.apache.org/maven2) @ line 99, column 25 -> [Help 2]
[ERROR] The build could not read 1 project -> [Help 1]
...

Network Access

• Maven Central responded, but the requested BOM xyz.tcheeric:nostr-java-bom:1.1.1 is unavailable, preventing dependency resolution.

---

https://chatgpt.com/codex/tasks/task_b_68e337e2acfc83319e62250f0db94bc8

[codecov-commenter]

⚠️ JUnit XML file not found

The CLI was unable to find any JUnit XML files to upload.
For more help, visit our troubleshooting guide.