feat(site): generate random hex string (non-uuid) for template builder session ID - #27935

Session ids for template builder telemetry need to be hexadecimal strings, not UUIDs.

• creates a generateSessionId helper function which reuses our existing generateRandomString logic
• renames our existing generateRandomString function to be generateRandomBase64String
• adds tests for utils/random.ts

additional context from #27671 (comment):

I just re-read the RFC; initial functional requirement 1 is:

Clients must generate 16-byte session ID encoded as a 32-character hexadecimal string.

UUIDs don't conform--they include hyphens in addition to hexadecimal chars. So httpmw's validSessionID function (added in this PR) will return false for a UUID.

coder/site/src/pages/TemplateBuilder/TemplateBuilderPage.tsx

Line 33 in 5064006

const sessionId = useMemo(() => uuidv4(), []);

I believe we will need a helper function to generate session IDs for the template builder after all, like #27661, just without the hyphens