docs(providers): drop dead getPlaceholder mentions

[XiaoZ-0218]

Summary

• Remove dead getPlaceholder() mentions from provider docs (mssql, oracle, druid, elasticsearch, opensearch).
• Keep engine bind facts (@p1/@p2, :1, Druid ?, ES/OS refuse) and align table notes with clickhouse.md after fix(sql): bind the inline row edit's values, and quote the remaining interpolated ones (#290) #304.
• Does not touch the same Druid line-number work tracked in docs/providers/druid.md: replace 18 stale line-number citations with named citations #590.

Fixes #640

[cevheri]

Merging this. Thank you, and welcome.

The easy way to close #640 was to delete six lines. You did the harder and better thing: every site kept the fact and dropped only the dead name. @p1 and :1 are still what those drivers send, Druid still binds ?, and the search providers still refuse positional parameters, so a reader loses nothing. Copying clickhouse.md's shape for the three table rows was the right call.

What I checked: grep -rn getPlaceholder docs/ src/ returns nothing, the merge onto today's main is clean, the citation guard stays green, and your two search-doc paragraphs are byte-identical, which keeps elasticsearch.md and opensearch.md parallel the way the suite expects.

One thing worth knowing, and it is not a criticism of this PR. Nothing measures your fix: the suite never mentions getPlaceholder, so the name could come back and no test would go red. That is why these six sites went stale unnoticed in the first place. A citation is only safe once something fails when it stops being true, which is the whole idea behind tests/unit/provider-docs-monitoring-citations.test.ts.

The issue also under-counted the problem, and that was my mistake, not yours. docs/ADDING_A_PROVIDER.md still credits SQLBaseProvider with "placeholder style" and docs/DATABASE_PROVIDERS.md with "placeholder generation". My grep missed both because they paraphrase the name instead of writing it. I am filing that with the guard attached. If you want to take it, say so and it is yours.

Good first contribution.

[XiaoZ-0218]

Happy to take that follow-up when you file it.

[cevheri]

@XiaoZ-0218 take it: #647