[Bug] check_capability does not validate elicitation sub-capabilities (form/url)

Summary

Connection.check_capability() returns True when a client has URL-mode elicitation but the caller checks for form-mode elicitation — the sub-capability (ElicitationCapability.form / .url) is never inspected.

Root cause

In src/mcp/server/connection.py:340-341:

if capability.elicitation is not None and have.elicitation is None:
    return False

This only returns False when the client has no elicitation at all. It does not check individual sub-capabilities (form / url). Compare with sampling, which correctly checks sub-capabilities:

if capability.sampling is not None:
    if have.sampling is None:
        return False
    if capability.sampling.context is not None and have.sampling.context is None:
        return False
    if capability.sampling.tools is not None and have.sampling.tools is None:
        return False

Reproduction

# Client supports URL elicitation only
have = ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability()))

# Check for form elicitation — should be False (client does not have form)
want = ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability()))

Connection.from_envelope("2025-11-25", client_info, have).check_capability(want)  # Returns True (BUG)

Impact

• check_capability is public API (via ServerSession.check_client_capability)
• Currently no production callers use elicitation sub-capability checks — low immediate impact
• But once someone relies on it (e.g., checking if the client supports form elicitation before calling elicit_form), it will return wrong results

Additional gaps (lower priority)

• extensions — ClientCapabilities.extensions is not checked at all
• tasks — ClientCapabilities.tasks and sub-capabilities entirely unhandled

Proposed fix

Add form / url sub-capability checks, matching the sampling pattern:

if capability.elicitation is not None:
    if have.elicitation is None:
        return False
    if capability.elicitation.form is not None and have.elicitation.form is None:
        return False
    if capability.elicitation.url is not None and have.elicitation.url is None:
        return False

AI assistance: Bug discovered and analyzed with AI assistance (opencode).

从 src/mcp/server/connection.py 的第340-341行附近开始，将 elicitation 处理与现有的 sampling 检查进行比较。复现 issue 中 URL-only 与 form-only capabilities 的情况，然后验证匹配和不匹配的 form/url capabilities 是否在相关测试套件中返回预期结果。