[HLD][Port Mirror] Add direction support, remove in-place update, and YANG model updates

[Janetxxx]

Why I did it

Follow-up to #2296, this PR updates the design with refinements made as the feature was implemented:

• Direction support (RX/TX/BOTH): extend from ingress-only to per-direction sampling; add the EGRESS_SAMPLEPACKET_ENABLE / EGRESS_SAMPLE_MIRROR_SESSION port attributes (RX = ingress pair, TX = egress pair, BOTH = both) and update the SAI call sequence.
• Stricter configuration semantics: a SET on an existing session is rejected as a duplicate instead of updated in place (reconfigure = explicit delete + re-add), and a sampled session must specify a direction.
• Fail-closed on unsupported platforms: reject the session when the required capability is missing, instead of silently downgrading to full mirroring.
• Per-direction conflict detection (5.2.4): describe the symmetric, per-direction SamplePacket conflict between sampled mirroring and sFlow.
• YANG: relax the sample_rate range to 2..max.
• Flow diagram (Section 4): simplify to the create/delete workflow with the rejection path and per-direction bind/unbind steps.
• Test plan (9.3.1 / 9.3.2): expand and renumber VS and system test cases (TX/BOTH/LAG, duplicate rejection, sFlow conflict, inactive/no-route, route withdraw–reactivate, truncate-without-rate default)

[mssonicbld]

/azp run

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