testnet: validate indexer configuration at startup with clear fatal errors

[Depo-dev]

Related to #215. A misconfigured indexer that starts and then misbehaves is far worse than one that refuses to start.

Scope

• Validate required configuration at startup: RPC endpoint, database URL, network, start ledger.
• Fail fast with a message naming the offending variable and what was expected.
• Reject contradictory combinations, such as a start ledger inconsistent with the configured network.
• Log the resolved effective configuration at boot, with secrets redacted.

Done when

• Every invalid configuration produces an immediate, specific fatal error rather than a degraded run.

[obswrld]

@obswrld has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I would like to work on this Issue

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[10xwhoman]

@10xwhoman has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello i would love to work on this.

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[drips-wave]

Congratulations, @10xwhoman! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

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