fix(config): honor AEVS_RECEIPT_VISIBILITY env var fallback

[web3guru888]

fix(config): honor AEVS_RECEIPT_VISIBILITY env var fallback

What

configure(receipt_visibility=...) defaulted to the truthy string "private",
so the resolution expression

receipt_visibility or os.environ.get("AEVS_RECEIPT_VISIBILITY", "private")

always short-circuited on the parameter and never read the
AEVS_RECEIPT_VISIBILITY environment variable. The env var was effectively
dead code.

Why it matters

The documentation states the option falls back to the env var:

• docs/08-api-reference.md — "Falls back to environment variables for api_key, agent_id, and receipt_visibility"
• docs/06-receipt-verification.md — "You can also set this via the AEVS_RECEIPT_VISIBILITY environment variable."
• docs/03-configuration.md — lists AEVS_RECEIPT_VISIBILITY → receipt_visibility in the env-var table.

A user setting AEVS_RECEIPT_VISIBILITY=proof_only (e.g. to keep inputs/outputs
off the wire in production) silently got private instead — a privacy-relevant
surprise.

The fix

Default the parameter to None so the existing or expression resolves with
the intended precedence: explicit argument → AEVS_RECEIPT_VISIBILITY → "private".

-    receipt_visibility: str = "private",
+    receipt_visibility: str | None = None,

The resolution site is unchanged — None is falsy, so precedence now works as
documented. The docstring is updated. The "never raise to host code" contract is
preserved: an invalid value from either source still warns and falls back to
"private" via _sanitize_config.

Tests

New tests/test_config.py::TestReceiptVisibility (6 cases): default,
explicit-param, env fallback, explicit-param-beats-env, case-insensitivity, and
invalid-value warn-and-fallback. The env-fallback / case / invalid cases fail on
main and pass with this change.

Verification

make check on this branch:

• pytest: 498 passed, 2 skipped (492 baseline + 6 new)
• ruff check: clean
• mypy --strict: clean (22 files)

Closes the documented-vs-actual behavior gap for receipt_visibility.