docs(gateway): add kong.client.get_principal to sandboxing strict mode allowlist

[jijiechen]

Description

Kong Gateway added a new PDK function, kong.client.get_principal() (Kong/kong-ee#20892), and added it to the untrusted_lua = strict sandbox allowlist next to kong.client.get_consumer (lax inherits it automatically). This updates the hand-maintained allowlist mirror on the Lua sandboxing reference page to match, per the NOTE: Don't forget to update docs too! comment in the gateway's strict.lua/lax.lua source.

Fixes: N/A (no doc ticket filed; internal follow-up to Kong/kong-ee#20892)

Preview Links

N/A — single-line addition to an existing code block, no new page.

Checklist

• Tested how-to docs. If not, note why here. — N/A, this is a reference page code-block list, not a how-to.
• All pages contain metadata. — unchanged, no new page.
• Any new docs link to existing docs. — N/A, no new page.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager). — N/A, no autogenerated content touched.
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly. — unchanged formatting, matches surrounding list style.
• Every page has a description entry in frontmatter. — unchanged, no new page.
• Add new pages to the product documentation index (if applicable). — N/A, no new page.

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