fix: enforce string nip metadata

[tcheeric]

Summary

Related issue: #____

The NIP identifier associated with events and messages should follow the protocol definition that represents it as a string. The existing implementation still surfaced it as an integer, so callers could not rely on the string-based contract.

What changed?

• Update the shared IElement contract and message/event implementations to expose nip as a String.
• Align the generic event builder helpers with the string-based signature.
• Refresh the JSON parsing unit test to assert the new representation.

BREAKING

• None.

Review focus

• Confirm no remaining integer-based nip accessors are left in the event/message hierarchy.

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)

Testing

• 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 100, column 25
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]
[ERROR]   The project xyz.tcheeric:nostr-java:1.0.1-SNAPSHOT (/workspace/nostr-java/pom.xml) has 1 error
[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 100, column 25 -> [Help 2]

Limitations

• Build currently blocked by the unresolved xyz.tcheeric:nostr-java-bom:1.1.1 import.

Network Access

• No blocked domains encountered; Maven Central responded but did not host the requested BOM artifact.

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https://chatgpt.com/codex/tasks/task_b_68ec06d174548331a2b0365943ddea24