fix(target-health): scrub lastError URLs and fix unknown-state gaps

Combines three closely-coupled target_health.go fixes that all touch
targetHealthTracker.record/stateForLocked in the same commit, since
splitting them would leave an intermediate commit that fails to
compile or a test asserting behaviour its own commit does not yet
implement.

F1: record gains a rawURL parameter and scrubs err's text through
sanitizeProviderErr before storing lastError, so a target URL
configured with an embedded credential can never leak through GET
/admin/api/targets. The scrub runs before truncateRunes so a long
embedded URL cannot survive truncation unscrubbed. Every probe, the
per-server proxy hook (mcp_a2a.go), and both federation record
sites (mcp_federation.go) now pass their target's configured URL
through.

F3: maybeSweepTargetHealth re-checks the elapsed interval after
winning the single-flight CAS, closing a window where a caller's
own stale interval read could still win the CAS once a separate,
faster sweep had already started and finished within the same
interval.

F4: targetHealthEntry gains everOK, set on record's success branch.
stateForLocked now reads a target that has never once succeeded as
unknown rather than healthy, even below failureThreshold — the
threshold still protects against a single transient first failure,
it just does not promote a target this tracker has never seen
answer to healthy. targetHealthSnapshot gains observed so the admin
view can still tell apart never-observed (omit every other field)
from observed-but-never-succeeded (keep them) despite both reading
state unknown. The now-redundant lastOK field is removed.

Also included: target_health_test.go's own test-hygiene fixes
(F6) — an idempotent, t.Cleanup-registered unblock ahead of
srv.Close so a failing assertion cannot leave a released server
handler blocked until the test binary's own timeout, and an atomic
hit counter in the disabled-sweep test — since they land in the
same file as the tests above.

Claude-Session: https://claude.ai/code/session_01EebV5jyWvYocByNQ3oss1w

mcp_a2a.go

@@ -226,7 +226,7 @@ func (g *Gateway) handleTargetProxy(w http.ResponseWriter, r *http.Request, u *u
 	// success/failure rule.
 	start := time.Now()
 	result, ok := g.proxyUpstream(w, r, upstreamURL, g.targetClient, nil, nil, false, kind+" target (name "+name+")", g.targetTimeout)
-	g.recordTargetProxyHealth(kind, name, result, ok, time.Since(start))
+	g.recordTargetProxyHealth(kind, name, targetURL, result, ok, time.Since(start))
 }
 
 // validateTargetURLs checks every configured MCP-server and agent entry at