feat(adcp): support ADCP_BASE_URL env override in sync_schemas.py

[bokelley]

Closes #284

Summary

Adds ADCP_BASE_URL env var support to scripts/sync_schemas.py, matching the JS SDK (@adcp/client/scripts/sync-schemas.ts:23). Cross-SDK CI can now set the var once to point both clients at a fixture CDN or pre-release bundle server, without editing source.

• _ADCP_BASE = os.environ.get("ADCP_BASE_URL", "https://adcontextprotocol.org").rstrip("/") — default unchanged; trailing slashes stripped to prevent a doubled /protocol path segment that would silently 404
• BUNDLE_BASE_URL = _ADCP_BASE + "/protocol" — all three fetch sites (fetch_bundle, fetch_signature_sidecars) pick up the override automatically
• Prints ! ADCP_BASE_URL override active: <base> when the override is active, before the fetch banner, using the established ! sigil convention
• Module docstring updated to list both ADCP_BASE_URL and ADCP_SKIP_SIGNATURE env vars together for discoverability
• 3 new tests: default value (env absent), override applied, trailing-slash stripped; all use monkeypatch + fresh module load so the test is correct even when ADCP_BASE_URL is set in the environment when pytest starts

What was tested

• pytest tests/test_sync_schemas.py — 18 passed
• pytest tests/ --ignore=tests/integration --ignore=tests/conformance/signing/test_ip_pinned_transport.py — 2180 passed, 21 skipped

Pre-PR review

• code-reviewer: approved — blocker (env-sensitive test_default_value) fixed; nits noted below
• dx-expert: approved — no blocker remains, no new blocker introduced

Nits surfaced (not fixed — reviewer discretion)

• No runtime guard if ADCP_BASE_URL includes the /protocol suffix (e.g. https://fixture.example.com/protocol → doubled path, silent 404). The docstring warns against it; a ValueError guard would be stronger.
• urlopen accepts file:// URIs via ADCP_BASE_URL; acceptable for a dev tool given the SHA-256 + Sigstore chain validates integrity, but worth noting.
• Fresh module names (sync_schemas_fresh, sync_schemas_fresh2) use a numeric suffix; request.node.name-derived names would be cleaner but not necessary.

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See adcp#3121
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Session: https://claude.ai/code/session_01KXfYPHYKr3R6EDqUbbo9ty

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