[Client] Fill elicitation defaults when accepting

[wWzZb]

A client accepting elicitation/create without explicit field values currently has to rebuild the request schema's defaults itself. The conformance client instead returned empty content, even though each supported definition already exposes default.

ElicitationSchema::extractDefaults() now provides the canonical field => default map, preserves falsey values such as 0 and false, and skips fields with no default. The conformance client uses that map when accepting. Coverage includes string, integer/number, enum, titled enum, and boolean definitions.

I left out the optional ElicitResult::accepted() factory because the helper and existing constructor cover the path without adding another public entry point.

The elicitation-sep1034-client-defaults runner still times out on current main before the handler runs: the server sends elicitation/create on the standalone SSE stream, while HttpTransport does not yet open the GET stream tracked by #327. Its expected-failure entry therefore stays in place. The 2026-07-28 sep-2322-client-request-state scenario, which exercises the same handler path, passes 5/5.

Checks:

• vendor/bin/phpunit — 1686 tests, 4377 assertions, 7 existing skips
• vendor/bin/phpstan analyse --memory-limit=-1
• vendor/bin/php-cs-fixer fix --dry-run --diff --verbose
• make docs
• composer validate --strict
• sep-2322-client-request-state conformance — 5/5

Fixes #328

[chr-hertel]

Thanks @wWzZb! minor rework while merging incl changelog entry