feat(form): reject empty field name in add_field_strict and add_file_strict

[nao1215]

Summary

multipartkit/form.add_field_strict and add_file_strict previously accepted an empty (or whitespace-only) name silently, producing a wire image with Content-Disposition: form-data; name="". RFC 7578 §4.2 requires the name parameter to be the field name itself; an empty value is implementation-defined at the receiver (skip, overwrite siblings keyed on "", or reject the body). The strict variants now reject it with the new Error(EmptyFieldName(value:)) variant on FormError, while the non-strict add_field / add_file keep their existing silent-accept behaviour for backward compatibility.

Changes

• src/multipartkit/form.gleam: add EmptyFieldName(value:) variant to FormError; add_field_strict and add_file_strict now bool.guard on string.trim(name) == "" ahead of the existing CR/LF/NUL check and return Error(EmptyFieldName(value: name)); doc-comments on all four builders updated to spell out the RFC 7578 §4.2 non-empty name requirement and the strict-variant escape hatch.
• test/regression_empty_field_name_test.gleam: new regression suite covering empty / whitespace-only / tab-only name rejection for both strict builders, empty filename still allowed by add_file_strict, non-empty name still works, and a backward-compatibility pin that non-strict add_field("", _) still silently accepts.
• CHANGELOG.md: ## [Unreleased] / ### Added entry for the new variant and the strict-builder behaviour.

Design Decisions

Issue #51 listed three approaches (A: strict-Result reject, B: panic, C: docs-only). Approach A on the strict variants only — leaving non-strict behaviour intact — was chosen because the strict variants were introduced for exactly this kind of typed-error surfacing (#40, #41) and the non-strict variants have a documented silent-accept contract that callers may already rely on; a breaking change there would force every existing caller to migrate without a way to opt back into the old shape. The string.trim(name) == "" predicate covers both the bare empty string and whitespace-only inputs in a single branch, matching the Issue's acceptance criteria (" " must also reject). The empty-name guard runs before the control-byte guard so that a caller passing "" sees EmptyFieldName rather than the (technically also true) NameContainsControlBytes with the same empty payload. filename is intentionally left allowed-empty in add_file_strict because RFC 7578 §4.2 only constrains name.

Closes #51

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 365c3121-6215-4f31-8d6c-0bd7d919dfc6

📥 Commits

Reviewing files that changed from the base of the PR and between ab187f8 and 2b3e9b6.

📒 Files selected for processing (3)

• CHANGELOG.md
• src/multipartkit/form.gleam
• test/regression_empty_field_name_test.gleam

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-51-form-add-field-empty-name

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