fix: validate collection name consistently across all MCP tool methods

[adityamparikh]

Summary

Only createCollection and the two schema-modification tools validated the collection name. checkHealth, getCollectionStats, search, the three index*Documents tools, getSchema and getSchemaResource accepted null or blank and failed downstream — and a null collection reaching SolrJ silently targets the client's default collection rather than reporting a bad argument.

The message was also already duplicated on main: a BLANK_COLLECTION_NAME_ERROR constant in CollectionService and a copied string literal in SchemaService's private requireCollection.

Changes

• Extract one shared ToolArguments.requireCollection(String) into the util package and route all eleven call sites through it, so the four services cannot drift apart. SchemaService's private duplicate is removed.
• Single message: "Collection name cannot be null or empty", aligned with the document-creator family in fix(indexing): reject blank document input consistently across formats #105. SchemaService's sibling requireNonEmpty helper is aligned to the same wording.

Coverage after this change:

Service	Methods now validated
CollectionService	createCollection*, getCollectionStats, checkHealth
IndexingService	indexJsonDocuments, indexCsvDocuments, indexXmlDocuments
SchemaService	getSchema, getSchemaResource, addFields, addFieldTypes
SearchService	search

* already validated on main; switched to the shared helper.

Why a runtime null check in @NullMarked code

The package is @NullMarked with NullAway wired as a build error, so this may look redundant. It isn't — NullAway is a closed-world compile-time analysis and only binds callers the compiler can see.

MCP tool methods are invoked reflectively by the Spring AI annotation runtime: AbstractMcpToolMethodCallback resolves each parameter with a plain lookup against the request's argument map and passes the result straight to buildTypedArgument, which returns null for a null input. A missing or null JSON value therefore arrives as null no matter what the annotations declare. @McpToolParam(required = true) only marks the parameter required in the advertised JSON schema; the server does not validate incoming arguments against it.

The rationale is recorded on ToolArguments so it doesn't get "cleaned up" later.

Test plan

• ./gradlew build passes
• New CollectionNameValidationTest (9 tests, all green) asserts every entry point against the shared constant rather than a copied literal — that is what keeps the services from drifting
• No regressions

Companion to #105, which applies the same treatment to document-input validation and uses the same message wording.

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