Feature/indexer fixes

[Almikefred]

Summary

This PR addresses four issues in the indexer's RPC handling: explicit detection of archived contract state, making the poll interval
env-configurable, validating JSON-RPC id correlation, and adding test coverage for the env_or helper.

Changes

#106 — Detect and handle Soroban state archival errors

• Added new IndexError::Archived variant to distinctly signal when contract state is archived
• Archival errors now trigger retries (treated as transient) rather than failing opaquely
• Added helper function to detect archival-related error messages

#107 — Make poll interval env-configurable

• Converted POLL_INTERVAL constant to poll_interval() function
• Reads RWA_POLL_INTERVAL env var (seconds), defaults to 10s
• Operators can now tune refresh cadence without recompiling

#105 — Validate JSON-RPC id in responses

• Added id field to RpcEnvelope struct
• Validates response id matches request id (1)
• Returns clear error on mismatch rather than silently ignoring

#108 — Add env_or helper unit tests

• Test coverage for env var set vs. unset behavior
• Ensures the config fallback helper works correctly

Notes

• Code-only delivery: no install/build/test/scripts run during implementation.
• Committer: Almikefred.

Closes #106, Closes #107, Closes #105, Closes #108

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