Add compiler error when single value event used with multiple data entries

[mootz12]

What

Adds a compiler error during derive_event that informs the user single value events can't have multiple data fields.

Why

The compiler error that surfaced if multiple data fields existed for a single value event was unclear.

error: expected one of `.`, `;`, `?`, `}`, or an operator, found keyword `self`
   --> soroban-sdk/src/tests/contract_event.rs:224:5
    |
224 |     #[contractevent(data_format = "single-value")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected one of `.`, `;`, `?`, `}`, or an operator
    |

The new error is:

error: data_format = "single-value" requires exactly 0 or 1 data fields, but found more
   --> soroban-sdk/src/tests/contract_event.rs:224:5
    |
224 |     #[contractevent(data_format = "single-value")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |

Fixes #1793

Known limitations

None

[Copilot]

Pull request overview

Improves the #[contractevent] procedural macro diagnostics by emitting a clear compile-time error when data_format = "single-value" is used with more than one non-topic (“data”) field, addressing the confusing parser error reported in #1793.

Changes:

• Makes the post-checkpoint darling error accumulator mutable so additional validation errors can be added later in codegen.
• Adds an explicit validation error for data_format = "single-value" when more than 1 data field is present.