Add retryable generic client requests

[floitsch]

Summary

• add generic Client.request overloads for replayable byte-array requests
• retry idempotent methods when a cached connection has gone stale, with an explicit override for other methods
• stop implicitly retrying buffered post, post-json, and post-form calls so non-idempotent requests are not replayed
• document the retry boundary for buffered and streaming bodies and cover the public POST helpers with regression tests
• synchronize stale-connection tests with explicit server-close signals instead of sleeps or timing assumptions

Testing

• toit analyze -Werror src/client.toit
• toit analyze -Werror --project-root tests tests/client-request-retry-test.toit
• event-driven retry regression test passed 10 consecutive stress runs
• all 15 locally available non-WebDriver tests passed
• WebDriver tests remain unavailable locally because chromedriver and geckodriver are not installed