fix(config): reject a solr.url SolrJ cannot connect to at bind time

[adityamparikh]

Problem

solr.url is bound into a record and handed to SolrJ. Nothing checks that it is a URL SolrJ can use, so a misconfigured deployment starts cleanly and fails at first request with an opaque client error. The easy mistake is omitting the scheme:

solr.url=localhost:8983
  → URI.create(...) parses scheme="localhost", host=null
  → SolrConfig normalizes the *path* by string concatenation, never noticing
  → SolrJ: "localhost:8983/solr/" is not a URL it can connect to

Change

SolrConfigurationProperties gets a compact constructor that rejects anything but an absolute http/https URL with a host, with a message that names the property, shows an example and echoes the offending value:

Accepted	Rejected
http://localhost:8983	localhost:8983 (scheme localhost, no host)
http://localhost:8983/solr/	solr.example.com, /solr (not absolute)
https://solr.internal:8983/custom/solr/	ftp://…, file://… (SolrJ cannot speak them)
http://solr:8983/solr/	not a url, http://, empty, blank, absent

Boot surfaces the IllegalArgumentException through its bind failure analyzer, so startup prints Failed to bind properties under 'solr' with the reason.

Why the constructor and not Bean Validation. Hibernate Validator is not on the classpath; adding it for one field means a new runtime dependency in the native image plus two classes for a custom constraint, and @NotBlank/@URL alone cannot express "http(s) with a host" (Hibernate's @URL accepts ftp:// and file://). A record's compact constructor is the plain-Java place for an invariant, and the binder invokes it reflectively with null when the property is absent, so the one hasText check is the guard for that boundary. No network I/O; native-safe as-is.

Path normalization (appending /solr/) is unchanged and still lives in SolrConfig.

Verification

SolrConfigurationPropertiesTest: 19 cases over accepted URLs, rejected values, the absent value, and two ApplicationContextRunner bind tests. Those two are @DisabledInNativeImage because that runner builds its context through a JDK dynamic proxy; the constructor itself runs natively. ./gradlew build on JDK 25 against main @ b4ffe18: green.

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