discovery: hold connMu when writing tabletHealthCheck health fields

[netliomax25-code]

Description

1. tabletHealthCheck.SimpleCopy (and connectionLocked) read Target, Serving, Stats, LastError and PrimaryTermStartTime under connMu, but processResponse, the timeout branch of checkConn, closeConnection and finalizeConn write those same fields without holding connMu.
2. SimpleCopy is reachable from another goroutine through HealthCheck.GetTabletHealthByAlias while the per-tablet checkConn goroutine is applying a streaming health update, so go test -race reports a read/write race on those fields.
3. Took connMu around the field writes at all four sites, releasing it before SimpleCopy and before any connection Close so the lock is never held across network IO.

The added regression test drives processResponse and GetTabletHealthByAlias concurrently and fails under -race without the change.

Related Issue(s)

N/A

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

None. Internal locking fix with no user-visible behavior change.

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