Persist the tester cookie for 30 days so it survives browser restarts

[jevansnyc]

Fixes #1136

Problem

GET /_ts/set-tester minted the ts-tester cookie without Max-Age or Expires, making it a session cookie. Safari deletes session cookies when the application quits, so Safari testers lost the cookie on every restart and silently fell back to the baseline arm on their next visit. Chrome and Firefox mask the defect because session restore resurrects session cookies.

Change

• Add TESTER_COOKIE_MAX_AGE_SECONDS (30 days, 2592000) and append Max-Age to the tester cookie in format_tester_cookie
• Update the two tests that assert the exact Set-Cookie string (core tester_cookie test, fastly dispatch_set_tester test)

/_ts/clear-tester already expires the cookie explicitly with Max-Age=0 and is unchanged. The module documents that the cookie only affects tester routing and must not gate sensitive behavior, so the bounded persistent lifetime does not change its security posture.

Testing

• Test-first: updated both assertions, watched both fail, then implemented
• cargo test-fastly: 170 adapter + 2357 core tests pass
• cargo test-axum: pass
• cargo fmt --all -- --check, cargo clippy-fastly, cargo clippy-axum: clean

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