Backport: feat(mcp): allow custom fetch for HTTP and SSE transports

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This is an automated backport of #14134 to the release-v6.0 branch. FYI @jaydeep-pipaliya
This backport has conflicts that need to be resolved manually.

git cherry-pick output

Auto-merging packages/mcp/src/tool/mcp-http-transport.test.ts
Auto-merging packages/mcp/src/tool/mcp-http-transport.ts
CONFLICT (content): Merge conflict in packages/mcp/src/tool/mcp-http-transport.ts
Auto-merging packages/mcp/src/tool/mcp-sse-transport.test.ts
Auto-merging packages/mcp/src/tool/mcp-sse-transport.ts
CONFLICT (content): Merge conflict in packages/mcp/src/tool/mcp-sse-transport.ts
Auto-merging packages/mcp/src/tool/mcp-transport.ts
error: could not apply a00d1d3f9... feat(mcp): allow custom fetch for HTTP and SSE transports (#14134)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

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🚀 Published in:

Package	Version
ai	6.0.148
@ai-sdk/angular	2.0.149
@ai-sdk/gateway	3.0.90
@ai-sdk/langchain	2.0.154
@ai-sdk/llamaindex	2.0.148
@ai-sdk/mcp	1.0.34
@ai-sdk/react	3.0.150
@ai-sdk/rsc	2.0.148
@ai-sdk/svelte	4.0.148
@ai-sdk/vue	3.0.148