Preserve fractional seconds in to_timestamp()

[fallintoplace]

Summary

to_timestamp(float8) was pushed down as fromUnixTimestamp(toInt64(...)), which truncated fractional Unix timestamps. Leave the built-in for PostgreSQL evaluation so subsecond precision is preserved.

The regression coverage checks that 2042323443.232 does not match 2042323443, and does match the fractional value.

Testing

• The extension builds and installs with -Werror.
• The to_timestamp section of make installcheck REGRESS=functions matches.
• This ClickHouse 26.7 image still has unrelated existing base64/base64url, JSON, and timeout expectation differences.

[theory]

This is not the correct solution. Switch to toDateTime64() instead.

Postgres:

david=# select to_timestamp(1284352323.32300) ;
        to_timestamp        
----------------------------
 2010-09-13 04:32:03.323+00
(1 row)

ClickHouse:

:) set date_time_output_format='iso'

:) select toDateTime64(1284352323.32300, 6, 'UTC');

   ┌─toDateTime64(⋯23, 6, 'UTC')─┐
1. │ 2010-09-13T04:32:03.323000Z │
   └─────────────────────────────┘

[theory]

Much better, thanks. Just needs a few tweaks.

[theory]

Are these not coming out the same? We want to make sure they're equivalent, not just non-NULL.

[theory]

Link to this PR.

[theory]

Do not change this

[theory]

We still need to be sure that epoch 0 is the same. Please restore.

[fallintoplace]

Thanks, fixed the remaining tweaks. I restored the epoch-0 coverage, changed the integer and float tests to verify equality, added the PR link to the changelog, and restored the historical changelog entry. I also rebased onto main and pushed commit 221d64d.

Please take another look when you have a chance.