Allow empty and null ToolContext in tool method invocation

[kuntal1461]

Problem

Fixes #6545

Two related but independent bugs prevent callers from expressing an optional or absent tool context:

Bug 1 — MethodToolCallback (spring-ai-model)

validateToolContextSupport() treated an empty ToolContext the same as a missing one, and threw IllegalArgumentException in both cases. This forced callers to inject fake placeholder entries (e.g. Map.of("foo", "bar")) just to satisfy the guard, even when no real context was needed:

// Workaround required before this fix:
Map<String, Object> toolContext = new HashMap<>();
if (orderNumber != null) {
    toolContext.put("orderNumber", orderNumber);
} else {
    toolContext.put("foo", "bar"); // fake entry to prevent exception
}

Bug 2 — DefaultChatClient (spring-ai-client-chat)

ChatClientRequestSpec.toolContext() rejected maps that contained null values via Assert.noNullElements(), making it impossible to represent optional context entries as null.

Changes

spring-ai-model — MethodToolCallback

• Remove validateToolContextSupport(). Empty context is valid; the tool method decides which keys it requires.
• In buildMethodArguments(), convert a null ToolContext to new ToolContext(Map.of()) so tool methods that declare the parameter never receive null, preventing NPE without throwing.
• Remove now-unused imports ClassUtils and CollectionUtils.

spring-ai-client-chat — DefaultChatClient

• Remove the Assert.noNullElements() guard on context map values only.
• The following guards are preserved unchanged:
  • null map reference → still rejected
  • null keys → still rejected
  • blank/empty string keys → still rejected

Behaviour matrix

Scenario	Before	After
Method declares ToolContext + null context	throws	tool receives ToolContext(Map.of())
Method declares ToolContext + empty context	throws	tool receives the empty context
Method declares ToolContext + non-empty context	works	works (unchanged)
Method does not declare ToolContext + any context	silently ignored	silently ignored (unchanged)
toolContext(map) with null value	throws	accepted
toolContext(null) map reference	throws	throws (unchanged)
toolContext(map) with null key	throws	throws (unchanged)

Tests

New: MethodToolCallbackToolContextTests — 13 tests covering:

• null context (implicit via call(input)) → tool receives empty ToolContext
• explicit null context → tool receives empty ToolContext
• empty HashMap and Map.of() are accepted
• single and multiple context entries pass through unchanged (backward compatibility)
• null value in context map is forwarded to the tool
• key case is preserved
• method without ToolContext param: null / empty / non-empty context are all silently ignored (original behaviour preserved)
• exception path with empty and null context wraps correctly in ToolExecutionException

Updated: DefaultChatClientTests — 2 tests renamed from *ThenThrow to *ThenSucceed to reflect the new allowed behaviour for null map values.

Test results

Module	Tests	Failures
spring-ai-model	829	0
spring-ai-client-chat	569	0