feat(lapis): distinguish a SILO timeout from SILO being unreachable

[corneliusroemer]

A timeout was reported as "Could not connect to silo", which is wrong and misleads whoever reads it. HttpTimeoutException means LAPIS did connect, sent the request and gave up waiting - SILO is reachable and may be answering other callers perfectly well. Only ConnectException means not reachable.

Related to #1866 in that the misleading exception came up while figuring out what caused info to return silo connection error.

SiloTimeoutException maps to 503, the same status the other SILO-unavailable paths use.

Also edits the message for an uncaught silo exception: the problem isn't necessarily connection failure, could be anything so message shouldn't assume.

PR Checklist

• All necessary documentation has been adapted.
• All necessary changes are explained in the llms.txt.
• The implemented feature is covered by an appropriate test.

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[Copilot]

🟡 Changes recommended

User-facing error messages now include full SILO URIs (host/details) that are returned in API responses, which can leak internal service addresses.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR improves SILO error reporting in the LAPIS service by distinguishing request/connection timeouts from true “SILO not reachable” scenarios, and by mapping timeouts to HTTP 503 to align with other SILO-unavailable cases.

Changes:

• Introduces SiloTimeoutException and throws it on HttpTimeoutException with a more specific timeout message.
• Updates “could not connect” messaging to be more accurate and less assumption-based for non-connect exceptions.
• Adds a MockServer-based test to verify slow /info responses trigger SiloTimeoutException.

File summaries

File	Description
lapis/src/main/kotlin/org/genspectrum/lapis/silo/SiloClient.kt	Adds timeout-specific exception and messaging; refines connect vs other failure messages.
lapis/src/main/kotlin/org/genspectrum/lapis/controller/ExceptionHandler.kt	Maps SiloTimeoutException to HTTP 503 with warning-level logging.
lapis/src/test/kotlin/org/genspectrum/lapis/silo/SiloClientTest.kt	Adds coverage ensuring slow /info triggers the new timeout exception.

Review details

Suppressed comments (2)

lapis/src/main/kotlin/org/genspectrum/lapis/silo/SiloClient.kt:257

• This generic SILO error message also embeds the full URI (including host). Since Throwable messages are returned in error responses (including 500s), consider removing host details from the message to avoid exposing internal topology to clients.

            val message = "Error talking to silo at $uri: ${exception::class} ${exception.message}"

lapis/src/main/kotlin/org/genspectrum/lapis/silo/SiloClient.kt:295

• The timeout message includes the full SILO URI (including host) and is returned to clients as the 503 body. Consider limiting it to the endpoint path to avoid leaking internal service addresses while still distinguishing timeout vs connect-timeout.

        val elapsedMillis = (System.nanoTime() - startedAtNanos) / 1_000_000
        return when (exception) {
            is HttpConnectTimeoutException -> "Timed out connecting to silo at $uri after ${elapsedMillis}ms"
            else -> "Timed out waiting for a response from silo at $uri after ${elapsedMillis}ms"
        }

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Lite

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