fix: make Remove patch op sound for flow collections and empty parents

[mostafa]

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Summary

Closes #2122. Reworks the Remove patch op so it is sound for flow collections and nested structures, addressing the soundness concern raised in #1020 and following the "re-think within yamlpath" direction of #1000. Relates to META #876.

Previously Remove deleted whole lines, which:

• corrupted flow mappings and sequences (removing b from {a: 1, b: 2} deleted the whole line), and
• left dangling null parents (removing the only key under env: left env: with no value).

The earlier flow handling in #1020 detected the container by sniffing for {/[ tokens, which is brittle because it cannot distinguish nesting like {[ ... ]} from [{ ... }].

Removal span computation now lives in yamlpath as Document::removal_span, which uses the parse tree rather than token heuristics:

• Dispatches on the value's actual container kind (block vs flow) as reported by tree-sitter, so nested mixtures like [{a: 1}, {b: 2}] and {x: [1, 2]} resolve correctly.
• For flow members, removes the element plus a single adjacent comma and the whitespace up to its neighbor, and collapses a sole member cleanly to {} or [].
• Resolves the element at its own lexical site, so a terminal alias is not followed: removing b: *x deletes b: *x rather than the anchor definition a: &x 1 it points to.
• Collapses dangling empty block parents upward to the first ancestor that still has other content. Flow containers are left as valid {}/[], and block containers only nest inside block containers, so collapse never crosses styles.

yamlpatch's Op::Remove is now a thin call into Document::removal_span.

Verified outputs match the (draft) expectations in #1000: foo: { a: a, b: b } with /foo/b removed gives foo: { a: a }; an empty key foo/baz removal preserves the rest; and foo: [1, 2, 3: {abc: def}] with /foo/2 removed gives foo: [1, 2].

Design note: #1000 leaned toward extending QueryMode::Pretty in query_node and deleting that span. I added a dedicated removal_span instead, because removal has needs that pretty extraction does not (separator handling, sole-member collapse, parent collapse) and query_pretty is shared with Op::Replace. Happy to fold this into query_pretty if you prefer.

Test Plan

• cargo test -p yamlpath -p yamlpatch (added snapshot coverage for block/flow mappings and sequences, nested flow collections, multiline flow, alias safety, and block parent collapse)
• cargo test -p zizmor --bins insecure_commands (the only current Op::Remove consumer)
• cargo fmt --check
• cargo clippy -- --deny warnings

[woodruffw]

Thanks @mostafa! I'm hoping to give this a review in the next day or so.