Encapsulate ConcurrentHashtable.SizeManager and relocate createBounded

[dougqh]

What Does This Do

Two small, uncontroversial ConcurrentHashtable API cleanups surfaced during review of #12367 (the first real external consumer of ConcurrentHashtable):

1. Encapsulate SizeManager. State.sizeManager was a public field, so a caller with a State reference could reach into SizeManager directly instead of going through the State-accepting static helpers (estimateSize, isFull, tryReserveOrEvict, evictOne, evictAll). That's now package-private. It was also missing a lock-free tryReserve(State) wrapper — the one operation an external caller doing custom find/reserve/insert (like LogCollector in Use ConcurrentHashtable for telemetry log #12367) actually needs — so callers no longer have to bypass encapsulation just to call it.
2. Relocate createBounded. Moved from the nested ConcurrentHashtable.State.createBounded(...) onto ConcurrentHashtable.createBounded(...) directly for a nicer call site. D1.createBounded/D2.createBounded now delegate to it.

Pure refactor — no behavior change. Existing tests updated only to call ConcurrentHashtable.createBounded instead of ConcurrentHashtable.State.createBounded.

Motivation

See the review thread on #12367 (comments from @dougqh):

• "I think there was an oversight on my part. I didn't really intend to expose anything on SizeManager directly. I guess I should fix that."
• "I'm wondering if it would look nicer to move createBounded onto ConcurrentHashtable." (bric3 agreed)

These are the least controversial items from that discussion; a couple of other, more design-heavy follow-ups (a lock-free find/matcher primitive avoiding boxed composite keys, and a doc note on the bucketAt/isFull composability hazard) are being tracked separately and are not part of this PR.

Contributor Checklist

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🤖 Generated with Claude Code

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.78 s	14.64 s	[+0.1%; +1.8%] (maybe worse)
startup:insecure-bank:tracing:Agent	13.59 s	13.72 s	[-1.7%; -0.2%] (maybe better)
startup:petclinic:appsec:Agent	16.97 s	16.87 s	[-0.3%; +1.5%] (no difference)
startup:petclinic:iast:Agent	16.40 s	16.86 s	[-7.2%; +1.7%] (no difference)
startup:petclinic:profiling:Agent	16.47 s	16.86 s	[-3.4%; -1.3%] (significantly better)
startup:petclinic:sca:Agent	16.87 s	16.59 s	[+0.6%; +2.8%] (maybe worse)
startup:petclinic:tracing:Agent	16.05 s	16.01 s	[-0.8%; +1.2%] (no difference)

Commit: 9d2f878b · CI Pipeline · Benchmarking Platform UI

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