parser/unparser: emit valid CEL for doubles in scientific notation

[moraneus]

strconv.FormatFloat with 'g' and precision -1 emits scientific notation for doubles of magnitude < 1e-4 or >= 1e+21 (e.g. "1e-05", "1e+30"). The previous guard appended ".0" whenever the formatted string lacked a "." so that the literal would parse as a double rather than an int — but scientific notation already produces a valid double literal, so appending ".0" produced invalid output like "1e-05.0" that the parser rejects.

Treat the exponent marker 'e'/'E' as also indicating a valid double literal, only appending ".0" when neither "." nor "e"/"E" is present.

Adds regression test cases covering small/large magnitudes, negative exponents, and uppercase 'E'.

Fixes #1325

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[jnthntatum]

/gcbrun

[jnthntatum]

@moraneus thanks for the fix!