chore: restore telemetry by default

[cragwolfe]

Summary

PR #4281 accidentally changed the existing library-load telemetry ping from default-on/opt-out to explicit opt-in. This PR resumes the existing library-load telemetry by default when unstructured is imported.

Changes

• Removes the explicit UNSTRUCTURED_TELEMETRY_ENABLED gate while preserving the telemetry endpoint, query payload keys and values, 10-second HTTP timeout, opt-out semantics, and best-effort failure suppression. The synchronous nvidia-smi GPU probe now uses subprocess.run with a 1.0-second timeout and stdin, stdout, and stderr directed to DEVNULL; launch, non-zero-exit, and timeout failures keep import non-fatal and send gpu="False".
• Users can still turn telemetry off before import by setting either DO_NOT_TRACK or SCARF_NO_ANALYTICS to any non-empty value after trimming whitespace; either variable takes precedence.
• Updates the README disclosure, outbound-connectivity tooling, focused telemetry coverage, version (0.26.0), and changelog.
• Forces DO_NOT_TRACK=1 in the root pytest conftest.py before ordinary collection, including when an inherited value is empty or whitespace-only; focused telemetry tests explicitly clear both opt-outs under mocks to prove default-on behavior.
• Passes DO_NOT_TRACK=1 after any optional env file in make docker-test, ensuring the packaged Docker test layout opts out before pytest collection without changing the production image.

Validation

• Dedicated regressions: 3 failures on the pre-fix implementation, then 3 passes after the bounded probe and forced test opt-out were applied.
• /opt/homebrew/opt/uv-wrapper/bin/uv run --locked --group test pytest -q --tb=short test_unstructured/test_telemetry.py — 21 passed
• make check — passed
• make check-version — passed
• git diff --check — passed
• shellcheck scripts/image/test-outbound-connectivity.sh scripts/image/test-all-outbound-connectivity-scenarios.sh — passed on the unchanged shell files earlier in this PR
• BCE Junior / Opus and GPT-5.6 Pro Oracle reviews completed; incorporated the bounded GPU-probe, inherited empty/whitespace, and packaged Docker test-hermeticity findings.

(authored by codex)

[cubic-dev-ai]

All reported issues were addressed across 10 files

<sub>Shadow auto-approve: would not auto-approve because issues were found.

Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 4 files (changes from recent commits).

<sub>Shadow auto-approve: would not auto-approve because issues were found.

Re-trigger cubic</sub>

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

<sub>Shadow auto-approve: would require human review. Restores telemetry to default-on/opt-out, expanding data collection to all users and reversing the current opt-in default. This is a privacy policy decision requiring human sign-off.

Re-trigger cubic</sub>

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

<sub>Shadow auto-approve: would require human review. Re-enables default-on library-load telemetry, expanding behavior exposure to every importer; a privacy/analytics policy decision requiring human sign-off.

Re-trigger cubic</sub>