feat: add comprehensive tests for built-in string functions

[alvinreal]

Summary

Adds comprehensive test coverage for all built-in string functions specified in #18, along with a small enhancement to join() and clippy fixes.

Changes

New integration tests (tests/string_functions.rs — 21 tests):
Covers every TDD scenario from the issue:

• join("a", "b", "c") → "abc" (variadic string concat)
• join(.first, " ", .last) with field values
• join(.tags, ",") array join with separator
• split("a,b,c", ",") → ["a", "b", "c"]
• split("hello", "x") → ["hello"] (no match)
• lower("HELLO") → "hello"
• lower("Already lower") → "already lower"
• upper("hello") → "HELLO"
• trim(" hello ") → "hello"
• trim("no-spaces") → "no-spaces"
• replace("hello world", "world", "rust") → "hello rust"
• replace("aaa", "a", "b") → "bbb" (all occurrences)
• len("hello") → 5, len([1,2,3]) → 3, len("") → 0
• coalesce(null, null, "found") → "found"
• coalesce("first", "second") → "first"
• coalesce(.missing, .name) → fallback to existing field
• Error on wrong arg count
• Nested: lower(trim(...)), upper(replace(...))

Enhanced join() function:

• Now supports variadic string concatenation: join("a", "b", "c") → "abc"
• Array join with separator still works: join(["a","b"], ",") → "a,b"

Additional unit tests in functions.rs:

• split_no_match, lower_already_lower, lower_non_string_coerces
• trim_no_spaces, replace_all_occurrences, len_empty_string, len_array_3
• coalesce_first_non_null, join_strings, join_with_field_values

Clippy fixes:

• Replaced redundant closure with function reference
• Replaced match single-pattern with if let
• Fixed approx_constant warnings (3.14 → 3.15 in tests)

Fixes #18