docs(spring-ai): replace obsolete setup with compatibility warning

[nik13]

Summary

Replace the obsolete Spring AI integration guide with an interim compatibility warning. The historical M4-built traceAI wrapper is not a supported Spring AI 1.1.x setup; NoSuchMethodError involving Message.getContent() has been reported (TH-7477).

• Remove the M4+ support claim, floating main-SNAPSHOT dependency blocks, provider table and obsolete wrapper examples.
• Distinguish the historical compilation target from tested runtime compatibility and a published replacement coordinate.
• Link the versioned Spring AI 1.1 observability and Spring Boot 3.5 tracing references, without promising provider parity or enabling sensitive prompt/completion logging.
• Preserve the canonical page and the legacy /docs/tracing/auto/spring-boot redirect.
• Add seven dependency-free source-contract checks (scripts/spring-ai-warning.test.mjs).

Relationship to the SDK fix

The SDK repair is future-agi/traceAI#204 (Spring AI 1.1.x compatibility). This page does not depend on it: it names no new version or artifact coordinate, and it is accurate for the artifacts published today. It can merge now so the docs stop giving an install path that fails on current Spring AI.

Follow-up once #204 is merged and a release coordinate is published and verified: replace the warning with the new installation steps (a separate docs PR).

Verification

On head 968602d, base main b61510c (branch is up to date with main, no conflicts):

• PR check "Link audit & redirect enforcement" passed (run).
• node --test scripts/spring-ai-warning.test.mjs: 7 passed (they failed on the previous page).
• Local astro build: 1181 pages, exit 0. The built page contains the warning callout, and /docs/tracing/auto/spring-boot is a redirect to the canonical page. The build's two route-conflict warnings (/docs/sdk/tracing, /docs/sdk/annotation-queues) are pre-existing and unrelated.
• audit-links: 0 broken navigation/content links; check-deleted-pages.mjs main: no pages deleted.
• Separate source reviewer and independent verifier found no blocking defects. Both versioned official reference URLs were fetched successfully.

Limits

• Documentation-only mitigation; not an SDK repair or release.
• The source-contract checks are run by hand; they are not wired into the PR workflow.
• Other pages still mention the Java Spring wrappers in passing (sdk/tracing/index, sdk/tracing/register, sdk/tracing/instrumentors, the traceAI integration index cards). They are left for the post-release docs update rather than changed here.

Video demo

None; the change is a static docs page. The rendered page was checked in a local preview.