fix(mcp): defer impl construction so Start's writeEnabled controls readonly

[SarthakB11]

Problem

mcp.New eagerly constructs the underlying *mcp.Server with
Instructions: instructions(s.readonly). At that point s.readonly
is whatever WithReadonly set it to (default false). Start later
reassigns s.readonly = !writeEnabled, but the Instructions string
is already baked into the impl.

cmd/mcp.go calls mcp.New(mcp.WithPrefix(cmdPrefix)) with no
WithReadonly, then client.StartMCPServer(cmd.Context(), writeEnabled).
writeEnabled defaults to false and only flips when FUNC_ENABLE_MCP_WRITE=1
is set. The default func mcp invocation therefore runs in readonly mode
but ships the non-readonly system prompt. instructions_warning.md is
silently dropped and the agent assumes it can deploy and delete.

TestInstructions did not catch this because newTestPairWithReadonly
threads WithReadonly into New, which already produces the correct
baked Instructions for that call path.

Fix

Move the mcp.NewServer call and the AddTool / AddResource
registration out of New into a buildImpl method. Start invokes
it after s.readonly = !writeEnabled, so the resolved value is the
one baked into Instructions. New's public surface and Options
behavior are unchanged.

Tests

TestInstructionsStartArgDrivesReadonly mirrors the cmd/mcp.go flow:
mcp.New is called without WithReadonly, then Start receives a
concrete writeEnabled. The two subtests (readonly / write)
assert that the readonly warning appears or is absent based on the
Start argument, not on what New saw at construction time.

make check and make test clean.

/kind bug

fix: default `func mcp` invocation now ships the readonly system prompt instead of the write-mode one

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[knative-prow]

Welcome @SarthakB11! It looks like this is your first PR to knative/func 🎉

[knative-prow]

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[gauron99]

This seems to be superseded by #3707 which addresses the same issue with a simpler approach.

[lkingland]

Thanks for the contribution, but closing this one as a duplicate