feat(validation): add ADCP_VALIDATION_MODE env var for TS-SDK parity

[bokelley]

Closes #385

Adds ADCP_VALIDATION_MODE=strict|warn|off to the validation resolution chain in client_hooks.py, matching the TS SDK contract (adcontextprotocol/adcp-client#694). Also fixes the false docstring in client.py that incorrectly claimed PYTHON_ENV / ENV / ENVIRONMENT are honored.

Precedence chain (highest → lowest):

1. Explicit ValidationHookConfig field (per-side override)
2. ADCP_VALIDATION_MODE env var — applies to both request and response sides
3. ADCP_ENV legacy fallback — response side only (production / prod → warn)
4. Hard defaults: requests "warn", responses "strict"

Changes:

• src/adcp/validation/client_hooks.py: adds _read_validation_mode_env() helper and _VALID_MODES constant; updates resolve_validation_modes() with the new precedence chain; invalid values raise ValueError at client-construction time
• src/adcp/client.py: docstring corrected — removes bogus PYTHON_ENV/ENV/ENVIRONMENT references, adds ADCP_VALIDATION_MODE documentation
• tests/test_schema_validation_client.py: 7 new tests covering all three valid values, case-insensitivity, invalid value raises, explicit config wins, env var wins over ADCP_ENV, unset keeps defaults, end-to-end through ADCPClient

What was tested:

• pytest tests/test_schema_validation_client.py — 15 passed (9 existing + 6 new parametrized + end-to-end)
• mypy src/adcp/validation/client_hooks.py src/adcp/client.py — no issues
• ruff check — all checks passed

Pre-PR review:

• code-reviewer: approved — no blockers; nit: _VALID_MODES duplicates ValidationMode literals (safe as-is); nit: invalid-value test directly calls _read_validation_mode_env() rather than exercising through resolve_validation_modes()
• ad-tech-protocol-expert: approved — non-breaking per spec; nit: ValidationHookConfig class docstring predates this change and doesn't mention ADCP_VALIDATION_MODE override authority; nit: or-chain is safe with current three-value enum but would be fragile if a falsy sentinel mode were added

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01Ecbp5qb8VsWmugvD9k6HPT

---

Generated by Claude Code