Don't mutate caller's headers dict in SendFrame.build

[lesnik512]

Problem

SendFrame.build assigned the caller's dict directly instead of copying it:

all_headers: SendHeaders = headers or {}
all_headers["destination"] = destination

So client.send(body, destination, headers=my_headers) and transaction.send(...) wrote destination, content-length, content-type and transaction back into the dict the caller passed in. A caller reusing one headers dict across sends — in a loop, or as a module-level default — silently accumulates another destination's headers, and a later send to a different destination carries leftovers if any key is not overwritten on that path (content-type and transaction are only set when non-None).

SubscribeFrame.build in the same file already copies, so this was an inconsistency rather than an intentional design.

Change

SendFrame.build now mirrors SubscribeFrame.build:

all_headers: SendHeaders = headers.copy() if headers else {}

A shallow copy is sufficient: SendHeaders values are str.

The # type: ignore needs typeddict-item added alongside assignment — assigning a dict[str, str] copy to a TypedDict-annotated name reports both codes, same as the existing SubscribeFrame.build line.

Tests

Added two regression tests in test_send.py, one for a plain client.send and one for transaction.send, each asserting the caller's dict is unchanged after the call. Both fail before the fix and pass after.

Verification

• uv run pytest packages/stompman --ignore=packages/stompman/test_stompman/test_integration.py — 292 passed, 1 skipped
• uv run mypy . — no issues in 50 source files
• uv run ruff check --no-fix packages/ — 22 pre-existing noqa-comments findings in files this PR does not touch; none in the changed files