fix: default gen_ai payload capture to off

[shreyas-n-harness]

Summary

• Flip gen_ai.payload_capture_enabled default from true to false in default.py — prompt/response content is now captured only on explicit opt-in (HARNESS_/HA_/AT_/TA_GEN_AI_PAYLOAD_CAPTURE_ENABLED=true or YAML config). Unset or empty env var resolves to capture-off, matching the privacy-control intent in genai_env.py.
• Accept HA_ alias for instrumentation opt-in flags: HARNESS_ENABLE_API / HARNESS_ENABLE_AI_* now resolve under both HARNESS_ and HA_ (HARNESS_ wins when both set). AT_/TA_ remain unsupported for these flags.
• Honor explicit false for ENABLE_CONSOLE_SPAN_EXPORTER: was a presence check (any value, including false, enabled the exporter); now strict boolean — only true enables it, resolved with HARNESS_ > HA_ > AT_ > TA_ precedence via new is_env_flag_enabled. Removes the unused is_env_var_present helper.
• Update tests and docs (sdk-quickstart.md, AGENTS.md).

Test plan

• Full suite: 260 passed (mysql/postgresql integration excluded — require live DB, pre-existing failures on main)
• New tests: HA_ alias enables opt-in flags, HARNESS_ wins over HA_, AT_/TA_ ignored; console exporter boolean resolution incl. explicit false, empty, legacy aliases, precedence
• QA: unset/empty HA_GEN_AI_PAYLOAD_CAPTURE_ENABLED → prompts not captured; true → captured; HARNESS_ENABLE_CONSOLE_SPAN_EXPORTER=false → no console spans; HA_ENABLE_AI_OPENAI=true → OpenAI instrumented

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