docs(postgrest): add upsert tsdoc examples and fix response/options

[7ttp]

Adds missing @example blocks to the upsert TSDoc in postgrest-js.

• Includes accurate response example for the upsert-with-constraints case
• Adds the missing onConflict option to the exact-count example
• Consolidates everything into a single TSDoc block on the implementation

This improves the accuracy of the upsert examples and aligns them with the actual behavior of the JS client.

🔍 Description

Updates the upsert TSDoc with accurate, typedoc-native examples and correct response data.
This makes the behavior clearer for consumers of the PostgREST client.

What changed?

• Added two @example blocks showcasing common upsert patterns
• Corrected the response object shown in the constraints example
• Ensured onConflict is present where required
• Unified the examples into the implementation TSDoc so Typedoc can render them properly

Why was this change needed?

The previous examples were outdated and did not reflect the actual behavior returned by the client.
This improves clarity for developers using upsert.

Related: supabase/supabase#40658 fixes supabase/supabase#40653

📸 Screenshots/Examples

N/A — all changes are within TSDoc.

🔄 Breaking changes

• This PR contains no breaking changes

📋 Checklist

• I have read the Contributing Guidelines
• My PR title follows the conventional commit format: <type>(<scope>): <description>
• I have run npx nx format to ensure consistent formatting
• I have updated comments/examples where relevant

📝 Additional notes

None.

[mandarini]

Thank you very much for this! Just this small change and I'll approve!

[7ttp]

@mandarini can u please check this up and approve!