fix: correct recursion depth tracking in agent validation

[opspawn]

Summary

Fixes #1287

The tState.with() method in adk_api_translator.go mutated shared state instead of creating independent copies. This caused two bugs:

1. Depth counter counted siblings instead of nesting depth: Each call to with() incremented the same depth field on the shared tState, so an agent with 11+ flat sibling agent tools would hit the MAX_DEPTH=10 recursion limit even though the actual nesting depth was only 1.

2. Visited agents list leaked across branches: append() on the shared slice meant that agents visited in one branch of the tool tree could appear as "visited" in unrelated branches, potentially causing false cycle detection in diamond-shaped agent graphs.

The Fix

Replace the mutating with() method with a copy-on-write version that returns a new tState with an independent depth counter and a copied visitedAgents slice.

Test Cases Added

• Flat list of 12 agent tools (all leaf agents) — PASSES (was failing before fix)
• Deep nesting of 10 levels — PASSES (within limit)
• Deep nesting of 12 levels — correctly FAILS with recursion error
• True cycle A->B->A — correctly FAILS with cycle detection
• Diamond pattern A->B, A->C, B->D, C->D — PASSES (shared agent D is not a cycle)

Test plan

• All new test cases pass
• All existing tests pass (golden test failures are pre-existing on main, unrelated to this change)
• Diff is minimal — only with() method changed + tests added

[Copilot]

Pull request overview

This PR fixes a critical bug in agent validation where the tState.with() method mutated shared state instead of creating independent copies, causing two issues: (1) depth counters incorrectly counted siblings instead of nesting depth, and (2) visited agents leaked across branches, potentially causing false cycle detection in diamond-shaped agent graphs.

Changes:

• Modified tState.with() to use copy-on-write semantics, returning a new tState with independent depth counter and copied visitedAgents slice
• Added comprehensive test coverage for flat sibling lists, deep nesting, cycle detection, and diamond patterns

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
go/internal/controller/translator/agent/adk_api_translator.go	Replaced mutating with() method with copy-on-write version to fix shared state bugs
go/internal/controller/translator/agent/adk_api_translator_test.go	Added comprehensive test suite covering flat lists (12 agents), deep nesting (10 and 12 levels), cycles, and diamond patterns

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[antonioberben]

Hi, @opspawn . I have faced the same issue this PR solves. AFAIK, if you merge with main, the kagent team will be able to progress this PR. Thanks!