feat(validation): add ADCP_VALIDATION_MODE env var for JS parity

[bokelley]

Context

The TS SDK exposes ADCP_VALIDATION_MODE=strict|warn|off as a single env-var override for client validation. The Python SDK only honors ADCP_ENV=prod to flip response default to warn, which is less granular and not parity with the TS SDK.

Additionally, the docstring at src/adcp/client.py:381-383 falsely claims PYTHON_ENV / ENV / ENVIRONMENT are honored — they are not. That doc lies.

Refs:

• src/adcp/validation/client_hooks.py:62-71 (resolution chain)
• src/adcp/client.py:381-383 (incorrect docstring)

Acceptance Criteria

• ADCP_VALIDATION_MODE read in client_hooks.py resolution chain, accepted values strict|warn|off
• Precedence documented: explicit kwarg > ADCP_VALIDATION_MODE > ADCP_ENV legacy fallback > default
• When set, applies to both request and response validation hooks (TS-parity behavior)
• Fix the misleading docstring at client.py:381-383 — remove the bogus PYTHON_ENV / ENV / ENVIRONMENT references
• Tests cover all three values + invalid value (raises clearly)
• CHANGELOG note that this matches the TS SDK env-var contract

[bokelley]

Triage

Classification: Feature request + Bug (false docstring)
Bucket(s): validation
Status: drafting-pr
Milestone: (omitted — no milestone specified in issue)

What the experts said:

• ad-tech-protocol-expert: non-breaking per spec; ADCP_VALIDATION_MODE applied to both sides is correct TS-parity; precedence chain ordering is sound
• code-reviewer: approved; cast(ValidationMode, ...) + _VALID_MODES pattern is correct; or-chain is safe with the current three-value enum

My take: All acceptance criteria met in draft PR #388 — env var reading, precedence chain, both sides, docstring fix, tests (all three modes + invalid + kwarg-wins + env-wins-over-ADCP_ENV). Non-breaking: ADCP_VALIDATION_MODE is additive; ValueError on invalid values replaces silent fallthrough which was never a supported contract.

PR summary: Adds _read_validation_mode_env() to client_hooks.py, updates resolve_validation_modes() with the new precedence chain, fixes the false PYTHON_ENV/ENV/ENVIRONMENT docstring in client.py, and adds 7 targeted tests.

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Triaged by Claude Code. Session: https://claude.ai/code/session_01Ecbp5qb8VsWmugvD9k6HPT

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Generated by Claude Code