Add custom-validation, etag-caching, restclient-basic-auth: Boot 4.1 API pass

Three companion modules verifying and rewriting the Boot 4.1.1 / Framework
7.0.9 story for three older articles: the javax->jakarta.validation namespace
fix plus Jakarta Validation 3.1 record-validation clarification, ETag/
conditional-request APIs re-verified unchanged plus the starter rename, and
RestTemplate Basic Auth rebuilt on RestClient with the exchange() trap called
out. 19 real passing tests generate every transcript quoted from the three
companion articles.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EQNA6DJ9VgCtW6zhCE8Xud

README.md

@@ -27,6 +27,9 @@ files.
 | [`db-migrations-expand-contract/`](db-migrations-expand-contract) | [Zero-Downtime Database Migrations: Expand-Contract in Practice with Spring Boot](https://ankurm.com/zero-downtime-database-migrations-expand-contract-spring-boot/) | a real 4-deploy rolling sequence against two live replicas with a load generator proving 99.98% success, H2's `AUTO_SERVER` single-point-of-failure trap, a `NOT NULL` constraint that fails every Stage 4 insert, and `ALTER TABLE` silently dropping a concurrently committed row with no exception thrown |
 | [`openapi-versioning/`](openapi-versioning) | [springdoc-openapi with Spring Boot 4.1: Generating, Customising and Versioning Your API Spec](https://ankurm.com/springdoc-openapi-spring-boot-4-1-versioning/) | springdoc 3.1.1 silently merging same-path, different-version handlers into one `oneOf` operation with an arbitrary `operationId`, a working per-version fix with `GroupedOpenApi` + `OpenApiCustomizer`, and the officially-versioning-supported functional-endpoint path turning out to document only one of two registered versions |
 | [`graphql-dataloader/`](graphql-dataloader) | [Spring GraphQL 2.0: Schema-First APIs, DataLoader Batching and Killing N+1](https://ankurm.com/) | a naive `@SchemaMapping` resolver measured at 21 SQL statements for 20 books versus a `@BatchMapping` resolver's flat 2, a dangling foreign key nulling an entire GraphQL response via non-null propagation identically under both resolver strategies, and two Spring Boot 4.1 packaging changes (`DataSourceAutoConfiguration`'s new package, Jackson 3 by default) hit along the way |
+| [`custom-validation/`](custom-validation) | [Custom Validation in Spring Boot: Beyond the Basics!](https://ankurm.com/custom-validation-in-spring-boot-beyond-the-basics/) | the `javax.validation` to `jakarta.validation` namespace fix Boot 3 already required, Jakarta Validation 3.1's record-validation clarification proven on both field- and class-level custom constraints, a record validation failure's empty 400 body by default, and what `spring.mvc.problemdetails.enabled` does and does not fix |
+| [`etag-caching/`](etag-caching) | [Mastering Cache Control with ETag in Spring Boot RESTful APIs](https://ankurm.com/etag-cache-control-rest-api-spring-boot/) | `spring-boot-starter-web`'s own POM now reading "deprecated in favor of spring-boot-starter-webmvc", `ShallowEtagHeaderFilter` and `WebRequest.checkNotModified()` re-verified unchanged on Spring Framework 7, deep cache vs shallow cache, and conditional `PUT` with `If-Match` as optimistic locking |
+| [`restclient-basic-auth/`](restclient-basic-auth) | [Spring Boot RestTemplate with Basic Auth: A Modern Guide](https://ankurm.com/spring-boot-resttemplate-with-basic-auth-a-modern-guide/) | RestClient with Basic Auth two ways against a real embedded server, `{noop}` passwords confirmed to emit no runtime warning at all, `spring-boot-starter-restclient` as its own required Boot 4 module, and the `RestClient.exchange()` trap covered in depth by the [RestTemplate to RestClient migration guide](https://ankurm.com/resttemplate-to-restclient-migration-guide/) |
 
 Articles whose text is kept here rather than only on the blog have it under
 `<directory>/post/` — `post.md` for the body and `meta.md` for the title, excerpt and