cluster-settings: SHOW CLUSTER SETTING rounds byte-size values, hiding configured precision

**Describe the problem**

For byte-size cluster settings, `SHOW CLUSTER SETTING` and the
`/_admin/v1/settings` HTTP API display a *rounded* value rather than
the exact configured byte count. The stored value is correct, but the
user-visible value is misleading.

This is particularly confusing for the new
`sql.status.active_query_text.max_bytes` setting in 26.3, which
validates inputs in the range `[1000 B, 64 KiB]` — i.e. the user is
expected to be able to configure values that are not round multiples
of KiB.

**To Reproduce**

```sql
> SET CLUSTER SETTING sql.status.active_query_text.max_bytes = '12345B';
SET CLUSTER SETTING

> SHOW CLUSTER SETTING sql.status.active_query_text.max_bytes;
sql.status.active_query_text.max_bytes
------------------------------------------
12 KiB

> SET CLUSTER SETTING sql.status.active_query_text.max_bytes = '65535B';
SET CLUSTER SETTING

> SHOW CLUSTER SETTING sql.status.active_query_text.max_bytes;
sql.status.active_query_text.max_bytes
------------------------------------------
64 KiB
```

The same display rounding affects the HTTP API:

```sh
$ curl 'http://.../_admin/v1/settings?keys=sql.status.active_query_text.max_bytes'
{
"keyValues": {
"sql.status.active_query_text.max_bytes": {
"value": "64 KiB", # actual stored value is 65535
...
```

Behavioral testing confirms the stored value is correct: a query of
~70 KiB submitted while the setting is `65535B` is truncated to exactly
65535 bytes (not 65536). So this is purely a display issue.

The exact value is only available via `crdb_internal.cluster_settings`,
which requires `allow_unsafe_internals = true`.

This issue is not new in 26.3 — it reproduces with any byte-size setting
(e.g. `bulkio.backup.file_size`) — but it is now more user-visible
because `sql.status.active_query_text.max_bytes` is a new public setting
that explicitly accepts sub-KiB precision.

**Expected behavior**

`SHOW CLUSTER SETTING` and the HTTP settings API should display the
exact configured value (e.g. `65535 B`) or a representation that
preserves the original precision (e.g. `65535 B (~64 KiB)`).

**Environment:**
- CockroachDB v26.3.0-beta.2-dev (built from `release-26.3` tip)
- Server OS: macOS, but reproducible anywhere
- Client app: `cockroach sql`

**Additional context**

Discovered while QA-testing the new
`sql.status.active_query_text.max_bytes` setting.

Jira issue: CRDB-65175

Start by tracing how byte-size cluster settings are formatted for `SHOW CLUSTER SETTING` and the `/_admin/v1/settings` HTTP API. The report reproduces the rounding with `sql.status.active_query_text.max_bytes` set to `12345B` and `65535B`; check the relevant tests and make sure both interfaces display the exact configured precision.

Written by the indexing model from the issue text.