refactor(tui): split MCP header helpers

[cyq1017]

Problem

crates/tui/src/mcp.rs still keeps HTTP header framing and custom-header filtering inline with transport code, which makes the MCP transport split in #3310 harder to review incrementally.

Change

• Move MCP HTTP default/header filtering helpers into mcp::headers
• Keep helper visibility scoped to the parent MCP module
• Preserve existing header behavior and tests

Verification

• cargo test -p codewhale-tui mcp -- --nocapture
• cargo fmt --check --all
• git diff --check
• cargo clippy -p codewhale-tui --all-targets --locked -- -D warnings -A clippy::uninlined_format_args -A clippy::too_many_arguments -A clippy::unnecessary_map_or -A clippy::assertions_on_constants

Part of #3310.

[greptile-apps]

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[gemini-code-assist]

Code Review

This pull request refactors MCP HTTP header handling by moving related helper functions and constants from crates/tui/src/mcp.rs into a new submodule crates/tui/src/mcp/headers.rs. The feedback suggests using explicit module path qualifiers (self::headers and super::headers) when importing from the new module to avoid potential name resolution conflicts with the external headers crate.

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[gemini-code-assist]

Using use headers::... can cause ambiguity or compilation errors if an external crate named headers is present in the dependency tree (which is a very common crate in the Rust ecosystem). It is safer and more idiomatic to use self::headers to explicitly refer to the local module.

Suggested change

	use headers::{apply_safe_custom_headers, with_default_mcp_http_headers};
	use self::headers::{apply_safe_custom_headers, with_default_mcp_http_headers};

[gemini-code-assist]

Similarly, inside the tests module, use headers::... should be qualified as super::headers::... to avoid name resolution conflicts with any external headers crate.

Suggested change

	use headers::{MCP_HTTP_ACCEPT, is_safe_custom_header};
	use super::headers::{MCP_HTTP_ACCEPT, is_safe_custom_header};

[greptile-apps]

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