fix(stdlib): validate tdef against runtime value and fix incorrect type_defs

[thomasqueirozb]

Summary

The test_function! macro now validates that the declared tdef is actually a superset of the kind of the runtime want value. This catches cases where type_def() returns a type too narrow to cover all values the function can produce.

The validation exposed real bugs across many stdlib functions, all of which are fixed here:

• merge: deep merge now correctly includes fields from both objects (previously lost to's sub-object fields)
• round: type_def() now returns float for float inputs, integer for integer inputs (was always integer)
• xxhash: type_def() now returns bytes | integer (was bytes-only, but XXH32/64/3-64 return integers)
• parse_tokens: array unknown type now includes null
• pop / slice: use resolve_constant to compute the exact output type when inputs are literals
• parse_regex / parse_regex_all: when numeric_groups=false (default), numeric-index capture groups are excluded from the type
• parse_syslog, parse_apache_log, parse_aws_alb_log, parse_aws_vpc_flow_log, parse_common_log, parse_nginx_log, parse_influxdb, parse_xml: field types corrected and optional fields marked with | undefined

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

Full cargo test -p vrl --lib passes (1846 tests, 0 failures).

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please run dd-rust-license-tool write and commit the changes. More details here.

References

NA