feat(contrib): add multi-agent orchestration patterns and samples

[svetanis]

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Link to Issue or Description of Change

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2. Or, if no issue exists, describe the change:

Problem:
Building complex systems with multiple agents requires choosing an orchestration strategy. Providing concrete examples of different layouts—like sequential, parallel, or hierarchical flows—helps demonstrate how these agents can work together effectively in a Java environment.

Solution:
This PR adds a set of multi-agent patterns to the contrib directory. It uses three practical scenarios (a code workflow, a report writer, and a trip advisor) to show how the ADK can be used to coordinate multiple agents for different types of tasks. These patterns leverage the ADK's native session state and web server.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Note: As this is a contribution to the contrib directory focused on sample implementations and patterns, formal unit test suites were not included. Verification was performed via the Manual E2E tests described below.

Manual End-to-End (E2E) Tests:

Verified all three projects via the ADK Web UI:

- Launched MultiAgentsSystem.java.

- Verified the Code Workflow handles iterative refinement loops.

- Verified the Report Writer correctly delegates to research sub-agents.

- Verified the Trip Advisor executes parallel searches and aggregates the itinerary.

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

This contribution includes 4 detailed README files located within the project directories to provide architectural clarity for each pattern. The implementations focus on native ADK integration for state and web rendering.