`Document.mutate()` shares the metadata map with the original document

Bug description

Document.mutate() is documented as "allowing selective modification without altering the original", but it passes the document's internal metadata map into the new builder by reference:

public Builder mutate() {
    return new Builder().id(this.id).text(this.text).media(this.media).metadata(this.metadata).score(this.score);
}

Builder.metadata(String key, Object value) then calls put directly on that map, so mutating the builder mutates the original Document's metadata.

Environment

• Spring AI 2.0.2-SNAPSHOT, main at 0933bd0
• Module: spring-ai-commons
• Class: org.springframework.ai.document.Document

Steps to reproduce

Document original = Document.builder().text("Original text").metadata("original", "value").build();

Document mutated = original.mutate().metadata("new", "metadata").build();

// fails: original now contains the "new" key as well
assertThat(original.getMetadata()).doesNotContainKey("new");

Expected behavior

mutate() seeds the builder with a copy of the metadata map (new HashMap<>(this.metadata)), so builder modifications never leak into the original document. Note that mutate() for the other fields already behaves this way because they are immutable or reassigned on build.

Start in the Document class in the spring-ai-commons module, focusing on mutate() and Builder.metadata(String, Object), then reproduce the issue with the example in the report. Done means builder metadata changes no longer appear in the original document, while the mutated document retains the new metadata.