fix(dbt): quote exported YAML scalars so values keep their type

[kevincostner17]

Summary

export_dbt_tests hand-writes schema.yml, and _scalar left strings unquoted when they would still change type under YAML 1.1 loaders (PyYAML, which dbt uses): 2024-01-01 loaded as a date, 0x1F as 31, 0b101 as 5. The regex's $ anchor also matched before a trailing newline, so "a\n" was written unquoted and lost its newline. Newlines inside quoted scalars were not escaped, so they folded to spaces.

• A string is written unquoted only if it fully matches a letter or underscore followed by [A-Za-z0-9_.-] and is not a bool/null/nan/inf word. Everything else is double-quoted.
• Inside double quotes, backslash, quote, \n, \t, \r, other C0/C1 controls, DEL, U+2028/U+2029, lone surrogates and U+FFFE/U+FFFF are escaped.
• Python floats now load back as floats: .nan / .inf / -.inf, and exponent forms get a mantissa dot (1e+16 becomes 1.0e+16).
• Real ints, floats and bools are still written as plain YAML numbers and bools. No new runtime dependency.

Tests

New tests/test_integrations/test_dbt_tests_exporter_yaml.py. Every value is loaded back with yaml.safe_load, both as a mapping value and as a list item, and must come back as the same value and type:

• Tricky strings: dates and timestamps, 12:30, 0x1F/0b101/0o17/017, 1e3, 1_000, .5, +1/-1, .inf/.NaN, yes/No/on/null/~/y/n, YAML indicators, leading/trailing spaces, quotes, backslashes, tabs, newlines, control characters, NEL, U+2028/U+2029, BOM, non-characters, non-ASCII.
• Ints, bools, None, and floats including NaN, ±inf, 1e16, 1e-5 and numpy floats.
• Every 1- and 2-character string over a tricky alphabet.
• Ordinary identifiers stay unquoted.
• The issue repro end-to-end through export_dbt_tests, plus a mixed accepted_values round trip.

Verification

• ruff check . clean; mypy src/freshdata clean
• pytest -m "not online and not large": py3.12 4374 passed; py3.9 4370 passed

Closes #342

[coderabbitai]

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[github-actions]

FreshData benchmark report — performance

• freshdata: ?
• python: ?
• platform: ?

fixture	n_rows	n_cols	p50 s	p95 s	peak MB	repair %	false-repair %	preserve %	trust	monotonic	export %

Authored-code reduction (Metric 6)