Upgrade Spring Boot to 3.4/3.5 and Jackson to 2.18.x

## Summary

The `conductor-client-spring` module currently pins Spring Boot to `3.3.0`. Spring Boot 3.3.x reached end-of-OSS-support in November 2024 and has known CVEs. Jackson is pinned to `2.17.1` across the project.

This issue tracks upgrading both to currently-supported versions.

## Scope

- [ ] Bump `spring-boot-starter` in `conductor-client-spring/build.gradle` from `3.3.0` → `3.4.x` or `3.5.x` (latest supported)
- [ ] Bump `jackson` version in `versions.gradle` from `2.17.1` → `2.18.x` (latest stable)
- [ ] Verify all modules build and tests pass after the upgrade
- [ ] Check for any API compatibility breaks between Spring Boot 3.3 → 3.4/3.5 affecting the SDK

## Context

- Spring Boot 3.3.x is EOL; open CVEs #84 and #85 are filed against it
- PR #86 bumps Spring Boot only to `3.3.11` (latest 3.3.x patch) — that is a stopgap, not the long-term fix
- Jackson 2.18.x is the current stable 2.x line; upgrading Spring Boot will also pull in a compatible Jackson via the BOM
- `jackson-module-afterburner` (currently used) was deprecated in Jackson 2.17 and removed in 2.18 — migration to `jackson-module-blackbird` will be required as part of this upgrade

## Migration note

`jackson-module-afterburner` → `jackson-module-blackbird` swap required in `build.gradle`:
```diff
-implementation "com.fasterxml.jackson.module:jackson-module-afterburner:${versions.jackson}"
+implementation "com.fasterxml.jackson.module:jackson-module-blackbird:${versions.jackson}"
```

Beginne mit conductor-client-spring/build.gradle und versions.gradle, um die aktuellen Deklarationen für Spring Boot, Jackson und Afterburner zu bestätigen. Prüfe die Kompatibilitätshinweise für Spring Boot 3.4.x oder 3.5.x und ersetze das veraltete Jackson-Modul wie angegeben. Baue anschließend alle Module und führe die Testsuite aus. Erledigt ist die Aufgabe, wenn die unterstützten Versionen angewendet sind, Kompatibilitätsprobleme behoben wurden und alle Builds und Tests erfolgreich sind.

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