Introduce entry-based DSL for JdbcClient batch updates #37341

Overview

The BatchSpec API introduced in 7.1 milestones for JdbcClient uses a method-chaining style in which parameters are bound to a "current" batch entry and consecutive entries are separated via add():

client.sql("INSERT INTO user (first_name, last_name) VALUES (:firstName, :lastName)")
    .batch()
        .param("firstName", "Jane").param("lastName", "Smith").add()
        .param("firstName", "John").param("lastName", "Doe")
    .update();

This style is error-prone: a missing add() still compiles but silently merges parameters into a single entry. It also does not lend itself to the typical batch use case of providing parameters dynamically — for example, from an existing collection of domain objects.

Since BatchSpec has not been released yet, we should revise its design before 7.1 RC1.

Proposal

Replace the chaining style with self-contained batch entries:

• BatchSpec :: entry(Consumer<BatchEntry>): defines one entry via a callback.
• BatchSpec :: entry(List<?>) and entry(Map<String, ?>): define one entry with positional or named parameters.
• BatchSpec :: entries(List<?>) and entries(Object...): define one entry per parameter object (DTO, record, Map, or SqlParameterSource).
• New BatchEntry type exposing only the singular param(...) and paramSource(...) methods.
• Remove add() as well as the param(...), params(...), and paramSource(...) methods from BatchSpec.

entry(...) and entries(...) calls can be freely combined within the same batch.

client.sql("INSERT INTO user (first_name, last_name) VALUES (:firstName, :lastName)")
    .batch()
        .entry(entry -> entry.param("firstName", "Jane").param("lastName", "Smith"))
        .entry(Map.of("firstName", "John", "lastName", "Doe"))
    .update();

List<User> users = ...;
client.sql("INSERT INTO user (first_name, last_name) VALUES (:firstName, :lastName)")
    .batch()
        .entries(users)
    .update();

Related Issues

• Add batch update support to JdbcClient #37216
• Improve JdbcClient to align BatchSpec with StatementSpec #37307