HTTPCLIENT-1482: expect-continue only on reused connections

[arturobernalg]

This change introduces an opt-in strategy that limits automatic emission of the Expect: 100-continue header to requests executed over a reused (pooled) connection.
It eliminates “No response from server” ambiguity on non-idempotent requests without imposing the extra RTT on brand-new sockets.
Key points

ExpectContinueTrigger enum – two modes: ALWAYS (default, preserves 5.x behaviour) and IF_REUSED.

RequestConfig extension – new field, getter, and builder setter setExpectContinueTrigger(...).

RequestExpectContinue interceptor – now consults the trigger and skips the header when the connection’s EndpointDetails#getRequestCount() is 0.

No binary incompatibility; existing code continues to operate exactly as before.

Unit tests added for both trigger modes.

The behaviour requested in Jira ticket HTTPCLIENT-1482 is now available to callers while keeping the classic client’s default semantics unchanged.

[ok2c]

@arturobernalg It would be good to add tests to TestRequestExpectContinue for the new config options.

[arturobernalg]

TestRequestExpectContinue

@ok2c my bad. I should have added tests Please take a look

[ok2c]

@arturobernalg Looks good to me.