fix(index,core): add depth-cap guards against unbounded recursion DoS

[bug-ops]

Summary

Two independent uncontrolled-recursion stack-overflow DoS bugs (CWE-674), same defect class and fix pattern, fixed together:

• chunk_children (crates/zeph-index/src/chunker.rs) recursed into AST child nodes with no depth limit — an attacker-crafted deeply-nested source file placed in a watched/indexed project directory could exhaust the native call stack and abort the whole process.
• collect_strings (crates/zeph-core/src/agent/tool_execution/tool_call_dag.rs) recursed over a tool call's input JSON with no depth limit — a deeply nested LLM-generated tool-call argument (e.g. steered by prompt-injected content the model read) could do the same on a more directly reachable surface.

A Rust stack overflow aborts the process uncatchably — it cannot be caught by catch_unwind, Result, or any async task boundary — so both are genuine full-DoS vectors, not just failures of the triggering task.

Fix

Both functions now take an explicit depth: usize parameter bounded by a module-level const:

• MAX_CHUNK_DEPTH = 512 — at the bound, chunk_children emits the oversized subtree as a single leaf chunk instead of recursing further (no content lost, just coarser chunk granularity for that subtree).
• MAX_JSON_DEPTH = 256 — at the bound, collect_strings stops descending into that branch (dependency detection just misses strings past the bound rather than crashing).

Neither path panics when the bound is hit; both log a tracing::warn! so operators get a signal that adversarial nesting was encountered in production.

Test plan

• 11 new regression tests (4 chunker, 7 DAG walker): depth-boundary exactness, graceful degradation under attacker-scale nesting (2000 levels / 612-level real tree-sitter-parseable fixture), and that nesting depth (not element count/width) is what's bounded — a 10,000-element flat array/object is unaffected.
• cargo +nightly fmt --check — clean
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings — clean
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins — 14909 passed, 36 skipped, no regressions
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler" — clean
• gitleaks protect --staged — no leaks

Follow-up

Filed #6594 to track the same unguarded-recursion pattern in two sibling functions (zeph-sanitizer/src/exfiltration.rs, zeph-tools/src/verifier.rs) that are out of scope for this PR — currently bounded in practice by serde_json's own default recursion limit on the upstream parse, but latent risk if that ever changes.

Closes #6551
Closes #6554